Re-add Mobile Specific section in .htaccess

This was removed in c6fdf126a4490f58b48ebaa184e4cc495359bf60.
Add missing period in README file
Remove need to readjust margins in `404.html`

Since some of the users prefer to just modify the placeholder 404
page instead of replacing it with their own, this commit makes it
easier to do that by allowing them to modify the content without
worrying about readjusting the margins.

Ref h5bp/html5-boilerplate#1567
Close h5bp/mobile-boilerplate#219
fixing conflicts