Clarify what should be committed in README

I understand the docs need lots of work[1] but I think
this line should be updated soon to include the decision
made in #123, even if a big rewrite is coming.  It has
confused me[2] and several other people[3].

[1]: https://github.com/tools/godep/issues/122#issuecomment-56259713
[2]: https://github.com/tools/godep/issues/131#issuecomment-59034488
[3]: http://stackoverflow.com/questions/26334220/should-i-commit-godeps-workspace-or-is-godeps-json-enough/26342342#26342342

fixes #131
closes #135