Add instructions on how to use sqlx migrations

[cypressf]

If someone wants to use sqlx instead of Diesel, it would be nice to see instructions on how to do that in the README.

[wileymc]

+1 I also need to figure this out. I'm thinking I'll have to write my own buildpack similar to this one with a prebuild script that runs sqlx migrate run before building. Let me know if you figure it out!