Clarify what should be committed in README

I understand the [docs need lots of work](https://github.com/tools/godep/issues/122#issuecomment-56259713) 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](https://github.com/tools/godep/issues/131#issuecomment-59034488) and [several other people](http://stackoverflow.com/questions/26334220/should-i-commit-godeps-workspace-or-is-godeps-json-enough/26342342#26342342).