docs: document a Python 3 migration plan

Bug: https://crbug.com/gerrit/10418
Change-Id: I72d82ce3a2d9af45d942bb10de82340110864ea5

docs/python-support.md

@@ -0,0 +1,32 @@
+# Supported Python Versions
+
+With Python 2.7 officially going EOL on [01 Jan 2020](https://pythonclock.org/),
+we need a support plan for the repo project itself.
+Inevitably, there will be a long tail of users who still want to use Python 2 on
+their old LTS/corp systems and have little power to change the system.
+
+## Summary
+
+* Python 3.6 (released Dec 2016) is required by default starting with repo-1.14.
+* Older versions of Python (e.g. v2.7) may use the legacy feature-frozen branch
+  based on repo-1.13.
+
+## Overview
+
+We provide a branch for Python 2 users that is feature-frozen.
+Bugfixes may be added on a best-effort basis or from the community, but largely
+no new features will be added, nor is support guaranteed.
+
+Users can select this during `repo init` time via the [repo launcher].
+Otherwise the default branches (e.g. stable & master) will be used which will
+require Python 3.
+
+This means the [repo launcher] needs to support both Python 2 & Python 3, but
+since it doesn't import any other repo code, this shouldn't be too problematic.
+
+The master branch will require Python 3.6 at a minimum.
+If the system has an older version of Python 3, then users will have to select
+the legacy Python 2 branch instead.
+
+
+[repo launcher]: ../repo