Update reviewing doc with py3 usage guidelines

Added reviewer guidelines and added a link to the main docs page.
Also adjusted the level of headings on the the main docs page so
they make sense in the "Page Contents" sidebar.

Change-Id: I476094ef14d31339afb76da484bef3fdc1be876e

doc/source/contributor/gerrit.rst

@@ -19,6 +19,44 @@ See `Getting Started`_ for information on how to get started using Gerrit.
 See `Development Workflow`_ for more detailed information on how to work with
 Gerrit.
 
+The Great Change
+----------------
+
+With the demise of Python 2.7 in January 2020, beginning with the Ussuri
+development cycle, Cinder only needs to support Python 3 runtimes (in
+particular, 3.6 and 3.7).  Thus we can begin to incorporate Python 3
+language features and remove Python 2 compatibility code.  At the same
+time, however, we are still supporting stable branches that must support
+Python 2.  Our biggest interaction with the stable branches is backporting
+bugfixes, where in the ideal case, we're just doing a simple cherry-pick of
+a commit from master to the stable branches.  You can see that there's some
+tension here.
+
+With that in mind, here are some guidelines for reviewers and developers
+that the Cinder community has agreed on during this phase where we want to
+write pure Python 3 but still must support Python 2 code.
+
+.. _transition-guidelines:
+
+Python 2 to Python 3 transition guidelines
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+* We need to be checking the code coverage of test cases very carefully so
+  that new code has excellent coverage.  The idea is that we want these
+  tests to fail when a backport is proposed to a stable branch and the
+  tests are run under Python 2 (if the code is using any Python-3-only
+  language features).
+* New features can use Python-3-only language constructs, but bugfixes
+  likely to be backported should be more conservative and write for
+  Python 2 compatibilty.
+* The code for drivers may continue to use the six compatibility library at
+  their discretion.
+* We will not remove six from mainline Cinder code that impacts the drivers
+  (for example, classes they inherit from).
+* We can remove six from code that doesn't impact drivers, keeping in mind
+  that backports may be more problematic, and hence making sure that we have
+  really good test coverage.
+
 Targeting Milestones
 --------------------
 

doc/source/index.rst

@@ -151,8 +151,13 @@ Contents:
    contributor/index
    API Microversions </contributor/api_microversion_dev/>
 
+For reviewers
+-------------
+
+* :ref:`transition-guidelines`
+
 Additional reference
-~~~~~~~~~~~~~~~~~~~~
+--------------------
 
 Contents:
 
@@ -165,7 +170,7 @@ Contents:
 .. only:: html
 
    Indices and tables
-   ~~~~~~~~~~~~~~~~~~
+   ------------------
 
    Contents: