Update doc/usage.md

HTML5 Boilerplate documentation link was incorrect.  
Merge pull request #687 from ShutterBrandon/patch-1

Update doc/usage.md
Merge commit '813f52bda679420947d5b3e42bb9f7d535459d13'
Merge remote-tracking branch 'remotes/roots/master'
modify entry-meta in attachment-image.php
add author/byline class
wrap entry-meta
add entry-taxonomy.php
don't ignore doc folder