Add entry for DrupalCon 2007 at the Yahoo campuss

[jowouters]

The Open Source CMS Summit was a two-day conference for those interested in open source content management systems. The conference was open to all open source content management systems, but in 2007 the vast majority of participants were there for Drupal and the conference morphed into DrupalCon 2007.

https://www.drupal.org/forum/general/events/2007-02-09/its-on-drupalcon-2007-at-yahoo