README.md: Fix link desc in Mac OS X prepration instructions
README.md: Add instructions for submodule init

The new crowsfoot theme is a submodule, and new cloners won't get
it locally unless they init and update.
Merge pull request #13 from alanorth/README

Update README.md
Merge branch 'master' of github.com:nairobilug/nairobilug.or.ke

* 'master' of github.com:nairobilug/nairobilug.or.ke:
  README.md: Add instructions for submodule init
  README.md: Fix link desc in Mac OS X prepration instructions
Merge branch 'master' into feature/cleanup

* master:
  README.md: Add instructions for submodule init
  README.md: Fix link desc in Mac OS X prepration instructions