add README for python-build.

was copied from [ruby-build](https://github.com/sstephenson/ruby-build).

.gitignore

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 /shims
 /version
 /versions
+/sources

plugins/python-build/README.md

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+# python-build
+
+python-build is a [pyenv](https://github.com/yyuu/pyenv) plugin
+that provides a `pyenv install` command to compile and install
+different versions of Python on UNIX-like systems.
+
+You can also use python-build without pyenv in environments where you
+need precise control over Python version installation.
+
+
+## Installation
+
+### Installing as an pyenv plugin (recommended)
+
+You need nothing to do since python-build is bundled with pyenv by
+default.
+
+### Installing as a standalone program (advanced)
+
+Installing python-build as a standalone program will give you access to
+the `python-build` command for precise control over Python version
+installation. If you have pyenv installed, you will also be able to
+use the `pyenv install` command.
+
+    git clone git://github.com/yyuu/pyenv.git
+    cd pyenv/plugins/python-build
+    ./install.sh
+
+This will install python-build into `/usr/local`. If you do not have
+write permission to `/usr/local`, you will need to run `sudo
+./install.sh` instead. You can install to a different prefix by
+setting the `PREFIX` environment variable.
+
+To update python-build after it has been installed, run `git pull` in
+your cloned copy of the repository, then re-run the install script.
+
+
+## Usage
+
+### Using `pyenv install` with pyenv
+
+To install a Python version for use with pyenv, run `pyenv install` with
+the exact name of the version you want to install. For example,
+
+    pyenv install 2.7.4
+
+Python versions will be installed into a directory of the same name
+under `~/.pyenv/versions`.
+
+To see a list of all available Python versions, run `pyenv install --list`.
+You may also tab-complete available Python
+versions if your pyenv installation is properly configured.
+
+### Using `python-build` standalone
+
+If you have installed python-build as a standalone program, you can use
+the `python-build` command to compile and install Python versions into
+specific locations.
+
+Run the `python-build` command with the exact name of the version you
+want to install and the full path where you want to install it. For
+example,
+
+    python-build 2.7.4 ~/local/python-2.7.4
+
+To see a list of all available Python versions, run `python-build
+--definitions`.
+
+Pass the `-v` or `--verbose` flag to `python-build` as the first
+argument to see what's happening under the hood.
+
+### Custom definitions
+
+Both `pyenv install` and `python-build` accept a path to a custom
+definition file in place of a version name. Custom definitions let you
+develop and install versions of Python that are not yet supported by
+python-build.
+
+See the [python-build built-in
+definitions](https://github.com/yyuu/pyenv/tree/master/plugins/python-build/share/python-build)
+as a starting point for custom definition files.
+
+### Special environment variables
+
+You can set certain environment variables to control the build
+process.
+
+* `TMPDIR` sets the location where python-build stores temporary files.
+* `PYTHON_BUILD_BUILD_PATH` sets the location in which sources are
+  downloaded and built. By default, this is a subdirectory of
+  `TMPDIR`.
+* `PYTHON_BUILD_CACHE_PATH`, if set, specifies a directory to use for
+  caching downloaded package files.
+* `PYTHON_BUILD_MIRROR_URL` overrides the default mirror URL root to one
+  of your choosing.
+* `PYTHON_BUILD_SKIP_MIRROR`, if set, forces python-build to download
+  packages from their original source URLs instead of using a mirror.
+* `CC` sets the path to the C compiler.
+* `CONFIGURE_OPTS` lets you pass additional options to `./configure`.
+* `MAKE` lets you override the command to use for `make`. Useful for
+  specifying GNU make (`gmake`) on some systems.
+* `MAKE_OPTS` (or `MAKEOPTS`) lets you pass additional options to
+  `make`.
+* `PYTHON_CONFIGURE_OPTS` and `PYTHON_MAKE_OPTS` allow you to specify
+  configure and make options for buildling MRI. These variables will
+  be passed to Python only, not any dependent packages (e.g. libyaml).
+
+### Checksum verification
+
+If you have the `md5`, `openssl`, or `md5sum` tool installed,
+python-build will automatically verify the MD5 checksum of each
+downloaded package before installing it.
+
+Checksums are optional and specified as anchors on the package URL in
+each definition. (All bundled definitions include checksums.)
+
+### Package download mirrors
+
+python-build will first attempt to download package files from a mirror
+hosted on Amazon CloudFront. If a package is not available on the
+mirror, if the mirror is down, or if the download is corrupt,
+python-build will fall back to the official URL specified in the
+defintion file.
+
+You can point python-build to another mirror by specifying the
+`PYTHON_BUILD_MIRROR_URL` environment variable--useful if you'd like to
+run your own local mirror, for example. Package mirror URLs are
+constructed by joining this variable with the MD5 checksum of the
+package file.
+
+If you don't have an MD5 program installed, python-build will skip the
+download mirror and use official URLs instead. You can force
+python-build to bypass the mirror by setting the
+`PYTHON_BUILD_SKIP_MIRROR` environment variable.
+
+### Package download caching
+
+You can instruct python-build to keep a local cache of downloaded
+package files by setting the `PYTHON_BUILD_CACHE_PATH` environment
+variable. When set, package files will be kept in this directory after
+the first successful download and reused by subsequent invocations of
+`python-build` and `pyenv install`.
+
+The `pyenv install` command defaults this path to `~/.pyenv/cache`, so
+in most cases you can enable download caching simply by creating that
+directory.
+
+### Keeping the build directory after installation
+
+Both `python-build` and `pyenv install` accept the `-k` or `--keep`
+flag, which tells python-build to keep the downloaded source after
+installation. This can be useful if you need to use `gdb` and
+`memprof` with Python.
+
+Source code will be kept in a parallel directory tree
+`~/.pyenv/sources` when using `--keep` with the `pyenv install`
+command. You should specify the location of the source code with the
+`PYTHON_BUILD_BUILD_PATH` environment variable when using `--keep` with
+`python-build`.
+
+
+## Getting Help
+
+Please see the [python-build
+wiki](https://github.com/yyuu/pyenv/wiki) for solutions to
+common problems.
+
+If you can't find an answer on the wiki, open an issue on the [issue
+tracker](https://github.com/yyuu/pyenv/issues). Be sure to
+include the full build log for build failures.
+
+
+### License
+
+(The MIT License)
+
+* Copyright (c) 2013 Yamashita, Yuu
+* Copyright (c) 2012 Sam Stephenson
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.