Create documentation on migrations

[mprahl]

There are a lot of pitfalls in migrations, for instance, if the migrations folder is not in sync with the alembic_version table in the database, errors will happen. Also, if a database migration goes wrong, documentation for a rollback should be provided.

[mprahl]

#93 sort of covers this, but we should keep this issue open past that PR.

[mprahl]

#138 should be able to close this issue.