Update readme: bundle install instructions, kramdown not maruku, links to Mou

Updated instructions for bundler. Clarified how to install ruby. Clarified how to run jekyll in the bundle, and how watching the filesystem works. Link to kramdown, since the site uses that, not maruku. Fix links to Mou. 
See my comment below -- running in the context of the bundle takes care of the jekyll version. I can put a note about github pages back in, though.