Make it clear in the README that MySQL is supported

[ccapndave]

Because only Postgres is mentioned by name, its not obvious that MySQL is supported.

[thiagomajesk]

@ccapndave Did you manage to use MySQL with this lib? There's this open issue about supporting MySQL enum so I'm not positive it's 100% supported. For example, I can't use create_type and type in my migrations, it just raises an ER_PARSE_ERROR. I'm still able to use it with string backed fields.