Add note regarding Respond.js use.

This note was recently added to the Bootstrap 3.0 docs:

Respond.js doesn't work with CSS that's referenced via @import. In particular, some Drupal configurations are known to use @import. See the Respond.js docs for details.

Here is the corresponding issue that added the note.

Respond.js is required for best support of IE8. In Drupal, "@import" statements are by default used when "CSS Aggregation" is disabled.

Should we add a note in our docs too? (Under the IE Compatibility section?)