Clarify location for Flyway callback scripts

[spilth]

This was frustrating us on a recent project as the Flyway documentation says to put your callback scripts in <install_dir>/sql when in fact they should also go in db/migrations.

[wilkinsona]

The Flyway documentation says (emphasis mine):

Using the default settings, Flyway looks in its default locations (<install_dir>/sql) for the Command-line tool"

<install_dir>/sql is just an (unfortunate) example that only applies to the CLI. That said, it could certainly be clearer on what those default locations are for other situations.

[wilkinsona]

Thanks for the PR. I've merged the change into master.