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. 
Are you certain that bundle is required? I just updated my local installation of Jekyll and build the docs just fine with the standard `jekyll serve` command. The Jekyll documentation also says to use `jekyll serve` without mentioning bundle. Could you elaborate a bit more about why we should change?