Added a note on submodules, closes #173.

[dblock]

I am also happy to contribute a more thorough CONTRIBUTING doc to replace this paragraph in the README ala https://github.com/ruby-grape/grape/blob/master/CONTRIBUTING.md, LMK if you would like that?

[digitalextremist]

Thanks @dblock. I wonder if by the time this change helps someone though, we won't be using the culture submodules any more. But the CONTRIBUTING.md sounds great, and the piece involving submodules could just be removed from that when it no longer applies.