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
Fix url to profile.png
Merge pull request #18 from Kirembu/master

Fix url to profile.png