Some documentation on how to write database migrations.

This commit sponsored by Andrei Cristian Petcu.  Thanks!

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+.. MediaGoblin Documentation
+
+   Written in 2011, 2012 by MediaGoblin contributors
+
+   To the extent possible under law, the author(s) have dedicated all
+   copyright and related and neighboring rights to this software to
+   the public domain worldwide. This software is distributed without
+   any warranty.
+
+   You should have received a copy of the CC0 Public Domain
+   Dedication along with this software. If not, see
+   <http://creativecommons.org/publicdomain/zero/1.0/>.
+
+==========
+Migrations
+==========
+
+So, about migrations.  Every time we change the way the database
+structure works, we need to add a migration so that people running
+older codebases can have their databases updated to the new structure
+when they run `./bin/gmg dbupdate`.
+
+There's a few things you need to know:
+
+- We use `sqlalchemy-migrate
+  <http://code.google.com/p/sqlalchemy-migrate/>`_.
+  See `their docs <https://sqlalchemy-migrate.readthedocs.org/>`_.
+- `Alembic <https://bitbucket.org/zzzeek/alembic>`_ might be a better
+  choice than sqlalchemy-migrate now or in the future, but we
+  originally decided not to use it because it didn't have sqlite
+  support.  It's not clear if that's changed.
+- SQLAlchemy has two parts to it, the ORM and the "core" interface.
+  We DO NOT use the ORM when running migrations.  Think about it: the
+  ORM is set up with an expectation that the models already reflect a
+  certain pattern.  But if a person is moving from their old patern
+  and are running tools to *get to* the current pattern, of course
+  their current database structure doesn't match the state of the ORM!
+- How to write migrations?  Maybe there will be a tutorial here in the
+  future... in the meanwhile, look at existing migrations in
+  `mediagoblin/db/migrations.py` and look in
+  `mediagoblin/tests/test_sql_migrations.py` for examples.
+- Common pattern: use `inspect_table` to get the current state
+  of the table before we run alterations on it.
+- Make sure you set the RegisterMigration to be the next migration in
+  order.
+- What happens if you're adding a *totally new* table?  In this case,
+  you should copy the table in entirety as it exists into
+  migrations.py then create the tables based off of that... see
+  add_collection_tables.  This is easier than reproducing the SQL by
+  hand.
+- If you're writing a feature branch, you don't need to keep adding
+  migrations every time you change things around if your database
+  structure is in flux.  Just alter your migrations so that they're
+  correct for the merge into master.
+
+That's it for now!  Good luck!