Add how-to guide on "incremental migrations"

[cloutiertyler]

Instead of migrating an old table, just add a new table alongside it, like player_v_2. When you want to do an operation, first check if the v_2 table has a row. If it does, you've already "migrated" that row, so use it. If it doesn't, look for a row in the old table, migrate it, insert it into the v_2 table, and then do the op. Wow! It's almost like we had migrations, except we don't!

Write a doc that describes how to do this in more detail, with examples.