Add more info about db:setup task

[letmein]

Hi!

I spent a while trying to figure what should be in db:setup, until I found that little comment in rom-sql sources:

        # Global environment used for running migrations. You normally
        # set in the `db:setup` task with `ROM::SQL::RakeSupport.env = ROM.container(...)`
        # or something similar.

Can we add a line about ROM::SQL::RakeSupport.env to the migration guide?