Explain use of Duct in shebangs

I love to use Duct to create small ruby+bundler utilities and use them directly from the command line. To make it easier and skip mentioning duct everytime I've been using it on shebangs.

Initially I ran into problems trying to use it because ruby interpreter needs to find the "ruby" string in the shebang (http://devoh.com/blog/2012/11/local-environment-variables), hence the "# ruby". I thought that maybe other people was being bitten by this and would be nice to document it.
Bring back removed sentence
Add explanation for # ruby comment
Merge branch @aitor PR #1