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. 
From http://bundler.io/bundle_install.html : 
> Gems will be installed to your default system location for gems. If your system gems are stored in a root-owned location (such as in Mac OSX), bundle will ask for your root password to install them there.

My system is configured with rbenv, which keeps installed gems neat and tidy, but for non-ruby-devs, we could recommend `bundle install --path vendor/bundle` and also add vendor/bundle to the .gitignore. I'll do that now.