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. 
Also, the bit about making sure to have the right versions of everything is also important and useful. Many people have old or outdated versions of Jekyll, and then make pull requests that we can't accept because the pages don't render. Github now has a page that lists the versions it uses, maybe the link should be updated to this one: https://pages.github.com/versions/