Migrating from gorp

[rubenv]

I've ported sql-migrate [1] into a second version named modl-migrate [2].

The only difficulty I had when migrating over from gorp was the fact that NewDbMap is obligatory (whereas in gorp it's just a struct you're making). Not doing this gives a very unobvious panic.

Now that gorp has gone unmaintained, perhaps it'd be a good idea to add a "Migrating from gorp" section to the README?

[1] https://github.com/rubenv/sql-migrate
[2] https://github.com/rubenv/modl-migrate