README.md update for basic setup for arc.
README.md update for basic setup for arc.

Summary:
arcanist requires a few steps to get set up, put these at the
bottom of the README.md file to make it a little easier to get
started.

Reviewers: adamvy

Reviewed By: adamvy

Projects: #foam

Differential Revision: http://codereview.cc/D740

foam-framework/foam2-transitional