Add sphinxcontrib-versioning

- 'make versioned' target to build w/versions, and supporting 'make
  prep' target.
- Rework how checking out refs in repos are done in the makefile
- Change versioning method from VERSION file to conf.py
- use fork of sphinxconfig-versioning to deal with repos/ structure
- move OTHER_REPO_DOCS in Makefile so doc8 target requires that docs are
  checked out in repos/ .

Change-Id: I6f5d6eee324bbbe31b71e1af596f0d8feb8f8785
diff --git a/readme.rst b/readme.rst
index a0acf09..8486e23 100644
--- a/readme.rst
+++ b/readme.rst
@@ -32,6 +32,17 @@
 reStructuredText Primer:
 http://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html
 
+Creating new Versions of Docs
+-----------------------------
+
+To change the version shown on the built site, change the ``versions`` variable in ``conf.py``.
+
+There is a ``make versioned`` target which will build all versions published on
+the remote to ``_build``.  Note that we're using a fork of the upstream version
+of the ``sphinxcontrib-versioning`` plugin, as the original isn't actively
+maintianed, and we needed it to be able to handle the symlink checkouts of
+other repos that are incorporated.
+
 Creating Diagrams
 -----------------