Add CONTRIBUTING.md (#2287)

CONTRIBUTING.md

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+General guidance
+================
+
+* The usual principes of respecting existing conventions and making sure that your changes
+  are in line with the overall product design apply when contributing code to Pyenv.
+
+* We are limited to Bash 3.2 features
+
+  That's because that's the version shipped with MacOS.
+  (They didn't upgrade past it and switched to Zsh because later versions
+  are covered by GPLv3 which has additional restrictions unacceptable for Apple.)
+
+* Be extra careful when submitting logic specific for the Apple Silicon platform
+
+  As of this writing, Github Actions do not support it and only one team member has the necessary hardware.
+  So we may be unable to test your changes and may have to take your word for it.
+
+
+Formatting PRs
+==============
+
+We strive to keep commit history one-concern-per-commit to keep it meaningful and easy to follow.
+If a pull request (PR) addresses a single concern (the typical case), we usually squash commits
+from it together when merging so its commit history doesn't matter.
+If however a PR addresses multiple separate concerns, each of them should be presented as a separate commit.
+Adding multiple new Python releases of the same flavor is okay with either a single or multiple commits.
+
+
+Authoring installation scripts
+==============================
+
+Adding new Python release support
+---------------------------------
+
+The easiest way to add support for a new Python release is to copy the script from the previous one
+and adjust it as necessary. In many cases, just changing version numbers, URLs and hashes is enough.
+Do pay attention to other "magic numbers" that may be present in a script --
+e.g. the set of architectures and OS versions supported by a release -- since those change from time to time, too.
+
+Make sure to also copy any patches for the previous release that still apply to the new one.
+Typically, a patch no longer applies if it addresses a problem that's already fixed in the new release.
+
+For prereleases, we only create an entry for the latest prerelease in a specific version line.
+When submitting a newer prerelease, replace the older one.
+
+
+Adding version-specific fixes/patches
+-------------------------------------
+
+We accept fixes to issues in specific Python releases that prevent users from using them with Pyenv.
+
+In the default configuration for a Python release, we strive to provide as close to vanilla experience as practical,
+to maintain [the principle of the least surprise](https://en.wikipedia.org/wiki/Principle_of_least_astonishment).
+As such, any such fixes:
+
+* Must not break or degrade (e.g. disable features) the build in any of the environments that the release officially supports
+* Must not introduce incompatibilities with the vanilla release (including binary incompatibilities)
+* Should not patch things unnecessarily, to minimize the risk of the aforementioned undesirable side effects.
+  * E.g. if the fix is for a specific environment, its logic ought to only fire in this specific environment and not touch execution paths for other environments.
+  * As such, it's advisable to briefly explain in the PR what each added patch does and why it is necessary to fix the declared problem
+
+Generally, version-specific fixes belong in the scripts for the affected releases and/or patches for them -- this guarantees that their effect is limited to only those releases.
+
+<h3>Backporting upstream patches</h3>
+
+Usually, this is the easiest way to backport a fix for a problem that is fixed in a newer release.
+
+* Clone Python, check out the tag for the appropriate release and create a branch
+* Apply existing patches if there are any (with either `patch` or `git am`) and commit
+* Cherry-pick the upstream commit that fixes the problem in a newer release
+* Commit and `git format-patch`
+* Commit the generated patch file into Pyenv, test your changes and submit a PR
+
+
+Deprecation policy
+------------------
+
+We do not provide official support for EOL releases and environments or otherwise provide any kind of extended support for old Python releases.
+
+We do however accept fixes from interested parties that would allow running older, including EOL, releases in newer environments.
+In addition to the above general requirements for release-specific fixes,
+
+* Such a fix must not add maintenance burden (e.g. add new logic to `python-build` that has to be kept there indefinitely)
+  * Unless the added logic is useful for both EOL and non-EOL releases. In this case, it will be considered as being primarily an improvement for non-EOL releases.
+* We do not provide any guarantees from our side that any such fix works or will continue working going forward. It's up to the interested parties to maintain it.
+
+
+Advanced changes / adding new Python flavor support
+---------------------------------------------------
+
+An installation script is sourced from `python-build`. All installation scripts are based on the same logic:
+
+1. Select the source to download and other variable parameters as needed.
+
+   This includes showing an error if the user's environment (OS, architecture) is not supported by the release.
+   Binary releases that only officially support specific distro(s) typically show a warning in other distros instead.
+
+2. Run one of the `install_*` shell functions
+
+`install_*` shell functions defined in `python-build` install Python from different kinds of sources -- compressed package (binary or source), upstream installation script, VCS checkout. Pick one that's the most appropriate for your packaging.
+
+Each of them accepts a couple of function-specific arguments which are followed by arguments that constitute the build sequence. Each `<argument>` in the build sequence corresponds to the `install_*_<argument>` function in `python-build`. Check what's available and add any functions with logic specific to your flavor if needed.
+
+We strive to keep out of `python-build` parts of build logic that are release-specific and/or tend to change abruptly between releases -- e.g. sets of supported architectures and other software's versions. This results in logic duplication between installation scripts -- but since old releases never change once released, this doesn't really add to the maintenance burden. As a rule of thumb, `python-build` can host parts of logic that are expected to stay the same for an indefinite amount of time -- for an entire Python flavor or release line.