Formalise Python version support policy (#252)

In `README.md`, state that we aim to support every Python branch still
supported by the PSF, and state the plugin versions that added or
removed support for specific branches. The plugin versions listed here
are based on when each Python version gained native-speed coverage
tracing in please_pex, which is a reasonable approximation of when that
version was supported by the plugin.

Author: chrisnovakovic

README.md

@@ -78,3 +78,34 @@ UsePypi = True
 PipFlags = ""
 DisableVendorFlags = False
 ```
+
+## Compatibility
+
+This plugin is compatible with the same operating systems as Please itself:
+
+- Darwin (amd64, arm64)
+- FreeBSD (amd64)
+- Linux (amd64, arm64)
+
+and is compatible with the following Python versions:
+
+- 3.9
+- 3.10
+- 3.11 (as of python-rules v1.7.0)
+- 3.12 (as of python-rules v1.7.0)
+- 3.13 (as of python-rules v1.8.0)
+- 3.14 (as of python-rules v1.14.0)
+
+Whenever possible, we aim to ensure the plugin works with all of the
+[supported Python branches](https://devguide.python.org/versions/#supported-versions). Please
+[report](https://github.com/please-build/cc-rules/issues) any bugs you encounter when building,
+testing or running Python code under one of these versions.
+
+### Unsupported versions
+
+The following Python versions are no longer supported by this plugin:
+
+- 3.8 (last supported by python-rules v1.7.4)
+
+Outputs that this plugin generates may not run correctly under these Python versions, or may run
+with significantly reduced performance.