Add Python.patch Note to Contributing.md (#271)

Add notes about contributing changes to Python.patch.

Co-authored-by: Russell Keith-Magee <[email protected]>

Author: johnzhou721

CONTRIBUTING.md

@@ -2,7 +2,30 @@
 
 BeeWare <3's contributions!
 
-Please be aware, BeeWare operates under a Code of Conduct.
+Please be aware that BeeWare operates under a [Code of
+Conduct](https://beeware.org/community/behavior/code-of-conduct/).
 
-See [CONTRIBUTING to BeeWare](https://beeware.org/contributing) for details.
+See [CONTRIBUTING to BeeWare](https://beeware.org/contributing) for general
+project contribution guidelines.
 
+Unless a fix is version specific, PRs should genereally be made against the
+`main` branch of this repo, targeting the current development version of Python.
+Project maintainers will manage the process of backporting changes to older
+Python versions.
+
+## Changes to `Python.patch`
+
+Additional handling is required if you need to make modifications to the patch
+applied to Python sources (`patch/Python/Python.patch`).
+
+Any iOS or macOS-specific changes should be submitted to the [upstream CPython
+repository](https://github.com/python/cpython). macOS and iOS are both
+officially supported Python platforms, and the code distributed by this project
+for those platforms is unmodified from the official repository.
+
+Changes to to support other platforms can be included in this PR, but they must
+also be submitted as a pull request against the `MAJOR.MINOR-patched` branch on
+[the `freakboy3742` fork of the CPython
+repo](https://github.com/freakboy3742/cpython). This is required to ensure that
+any contributed changes can be easily reproduced in future patches as more
+changes are made.