Decouple migrations: Write docs on how to write backwards-compatible migrations

Created by: efritz

Partially implements Step 0 in RFC 469.

Write documentation on how to make backwards compatible changes to the database schema. This should be in the developer pages of user-facing docs (as a sibling of the high performance SQL guide). This should include a recipe book of how to make common changes (adding a column table, removing unused columns/tables, changing the format of data in a possibly destructive way, etc).