Update doc/usage.md

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

Update doc/usage.md
Root relative URLs
comment
added conditional
Fix (hack) for subdir installs
Merge pull request #688 from damianregan/relativeurls

Relative URL fixes
Merge remote-tracking branch 'upstream/master'