.col-offset-0 example addition to docs

Created by: nathanhleung

I was having a bit of trouble making a responsive column layout where -xs devices would have an offset while -sm devices would not. On a whim I decided to try adding a .col-sm-offset-0 class and it worked, however I had to search for a while since -offset-0 classes are undocumented. I think it would be really helpful in the future if this was added to the documentation. If you guys feel the same I can create a quick pull request on gh-pages with the addition.

Relevant code:

<div class="row">
  <div class="col-xs-6 col-sm-4">
  </div>
  <div class="col-xs-6 col-sm-4">
  </div>
  <div class="col-xs-6 col-xs-offset-3 col-sm-4 col-sm-offset-0">
  </div>
</div>