Update Readme.md for configuration with MySQL

Thanks for supporting also Play! version 3.0.x!
As I tried to make it run for MySql, I noticed that there is an additional dependency needed in `build.sbt`:
```
libraryDependencies += "org.flywaydb" % "flyway-mysql" % "9.16.3"
```

May you update the README to makes this clear for other users? The version must be in sync with what is used in `flyway-play` as otherwise it will not compile