Added XKCD explained. Kudos to attrib, dawehner, pcambra and tstoeckler.

XKCD cartoons can be difficult to understand for end-users. There is a community wiki site where readers collaborate on adding explanations of the cartoon, along with complete transcripts.

In this PR have participated actively @attrib, @dawehner, @pcambra and @tstoeckler during Drupal Developer Days Szeged 2014.