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. 
It's important to note that the site is hosted on github pages. Why: people regularly make pull requests where they add a plugin that we can't use because Github is building our site for us :/