This attempts to add a README.md to help create consistent migrations
that maximize our test coverage and do not include unnecessary
statements.
It also adds a feature to have an `-- OLD DATA --` section as well
as a `-- NEW DATA --` section so that we can fix mistakes made in
previous snapshots (like a row that was forgotten to be added when a
table was created) without editing them going forward.
Change-Id: I28a786f8ef163cae1de1bb08f61af1e1104b0a88
