Add details on configuring yui compress via gems vs commands

The example combined the using the gems with calling out to a system command causing a 

    sh: yuicompressor: command not found

instead of actually using the yuk-compress gem as we expected.  Took us a while to figure out what was wrong with the _config.yml so this hopefully with help clarify it for other people also.  This is a more wordy version of https://github.com/moshen/jekyll-asset_bundler/pull/19 to consider.