Add excerpt breakers for MySQL articles

The index page doesn't need to display the whole article. This adds an
Octopress/Excerpt-aware comment to truncate and include a 'Read On'
link.

Change-Id: If9c8fe2d6cf616823c22bcfc2dc55b0f20f1306f
Reviewed-on: https://gerrit.causes.com/22828
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>