Add release cycle info to docs (#1798)

Documents the release cycle.

doc/contrib/release.rst

-Release Process
-===============
+Releases
+********
+
+Release cycle
+=============
+
+0. release every 3 months (at time ``T``)
+1. ``T-11`` weeks: ``all`` add your favorite Issues to the next-rel column
+2. ``T-10`` weeks: ``Scrum Master`` prep dev meet (internal)
+
+   * Update/trim next-release column in Kanban
+   * Prepare agenda, include possible additions not covered by Kanban/Issues
+   * Add milestone tags (nextver, nextver+1, etc.)
+3. ``T-8`` weeks: ``Release Manager`` dev meet (external/public)
+
+   * Use Kanban as starter
+   * Move issues around based on input
+   * Add milestone tags, for this release or future releases
+4. ``T±0``: ``Release Manager`` release!
+5. ``T+1`` weeks: ``Scrum Master`` retrospective
+   
+   * set date for next release
+
+Procedure
+=========
 
 These notes enumerate the steps required every time we release a new
 version of Arbor.