.htaccess: Add comment about font MIME types

Browsers usually ignore the font MIME types and sniff the content but,
Chrome, shows a warning if TrueType / Opentype fonts aren't served with
the `application/x-font-ttf` / `font/opentype` MIME type.

References:
* h5bp/html5-boilerplate#1317
* http://mimesniff.spec.whatwg.org/#matching-a-font-type-pattern