Fizz shouldn't require any columns, like `id` or timestamps

[markbates]

Currently Fizz is enforcing an old Pop rule that id and timestamps columns are added. The enforcement of these rules should be done in gobuffalo/pop not Fizz.

[okdas]

Just FYI this conflicts with documentation.

[stanislas-m]

Migration done in Pop development branch, and the fix tool can migrate the fizz files. So we just need to change the Readme now.