[AIRFLOW-XXXX] Adds branching strategy to documentation (apache#7193)

(cherry picked from commit 5b0fb42)

CONTRIBUTING.rst

@@ -152,6 +152,21 @@ these guidelines:
 -   Adhere to guidelines for commit messages described in this `article <http://chris.beams.io/posts/git-commit/>`__.
     This makes the lives of those who come after you a lot easier.
 
+Airflow Git Branches
+====================
+
+All new development in Airflow happens in ``master`` branch. All PRs should target that branch.
+We also have ``v1-10-test`` branch which is used to test ``1.10.x`` series of Airflow and where committers
+(and only committers) cherry-pick selected commits from the master branch.
+Cherry-picking is done with -x flag.
+
+The ``v1-10-test`` branch might be broken at times during testing. Expect force-pushes there so
+committers should coordinate between themselves on who is working on the ``v1-10-test`` branch -
+usually those will be people with the role of the release manager.
+
+Once the branch is stable - the ``v1-10-stable`` branch is synchronized with ``v1-10-test``.
+The ``v1-10-stable`` branch is used to release ``1.10.x`` releases.
+
 Development Environments
 ========================