Add a page to the developer guide on Alembic migrations

This page gives a brief overview of how to create an Alembic
migration and follow the Sahara revision naming scheme.  It
includes some of the text from the README in the source tree.

Partial-Implements: blueprint update-docs-icehouse
Change-Id: Ib361c7f0eb78136170506d268f5042e78b232398
Merge "Add a page to the developer guide on Alembic migrations"