COMMANDS.md: Add commands from `pyenv --help`

COMMANDS.md

@@ -5,9 +5,12 @@ first argument.
 
 The most common subcommands are:
 
+* [`pyenv commands`](#pyenv-commands)
 * [`pyenv local`](#pyenv-local)
 * [`pyenv global`](#pyenv-global)
 * [`pyenv shell`](#pyenv-shell)
+* [`pyenv install`](#pyenv-install)
+* [`pyenv uninstall`](#pyenv-uninstall)
 * [`pyenv versions`](#pyenv-versions)
 * [`pyenv version`](#pyenv-version)
 * [`pyenv rehash`](#pyenv-rehash)
@@ -15,6 +18,10 @@ The most common subcommands are:
 * [`pyenv whence`](#pyenv-whence)
 
 
+## `pyenv commands`
+
+Lists all available pyenv commands.
+
 
 ## `pyenv local`
 
@@ -176,6 +183,16 @@ or, if you prefer 3.3.3 over 2.7.6,
     Python 3.3.3
 
 
+## `pyenv install`
+
+Install a Python version (using `python-build`).
+
+
+## `pyenv uninstall`
+
+Uninstall a specific Python version
+
+
 ## `pyenv versions`
 
 Lists all Python versions known to pyenv, and shows an asterisk next to