Document the Patch Tree Approach. (#281)

Author: johnzhou721

CONTRIBUTING.md

@@ -23,9 +23,22 @@ 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
+Changes to to support other platforms can be included in a PR for this repo, 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.
+
+Note that the `MAJOR.MINOR-patched` branch of that fork is maintained in the format
+of a *patch tree*, which is a branch that has an entirely linear sequence of
+commits applied on top of another branch (in the case of the fork, `MAJOR.MINOR`),
+each of which adding a new feature. Therefore, bug fixes to the patches applied
+*will* be merged, but their changes gets squashed into the commit applying the
+feature. Feature additions to the patch will be squashed into a single commit,
+and then merged.
+
+This also means that if another contributor on the fork gets a pull request merged
+into the fork, you must *rebase*, not merge, your changes on top of the newly pulled
+`MAJOR.MINOR-patched` branch, since the "history" of a patch tree branch might
+change in a way that is incompatible with merge commits.