Refs #332: Add doc for database backend
Refs #332: Allow importing rapidsms.backends.db.DatabaseBackend
Refs #332: Add note about db backend

Add note suggested by copelco about the db backend not likely
to be used in production.

Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
or "Fixes bug."  This convention matches up with commit messages generated
by commands like git merge and git revert.
Merge pull request #332 from rapidsms/doc/332-database-backend

Add doc for database backend