Add note about Ruby >= 2.0 escaping `'`

In 2012 as Ruby 2.0 was being developed it was requested that single quotes (') be escaped by `CGI.escapeHTML` which Mustache uses under the hood to do the safe insertion. This change was made in https://github.com/ruby/ruby/commit/c47cca2f
Merge pull request #172 from tpickett66/patch-1

Add note about Ruby >= 2.0 escaping `'`