Document how to add timestamps to the __EFMigrationsHistory table

**Problem:**
Sometimes migrations doesn't work perfectly for some reason. It's often a bit hard to debug why because you can't check when a certain migration was applied to that database. It could be because some migrations depend on each other, but it could also be because a migration created corrupt data and we need to track that back.

**Proposal:**
Add a timestamp column in the __EFMigrationsHistory table that inserts a `GETDATE()` on inserts.

**Possible issues:**
Some people might have added this through their own migrations before. So there could be a collision. I guess the script could possibly check that before creating the column or it could be configurable.