Review the importing styles from npm section

[hwillson]

Reference: meteor/meteor#6846

Quoting from meteor/meteor#6846 (comment):

Importing .css files from npm packages, using CSS @import syntax, is still an issue with the latest version of Meteor (1.5.2), and can definitely be a pain to work around. The Importing styles from npm section of the Guide leads people to believe this functionality is supported and should exist, but we realize it isn't working.

We should look into adjusting this section, to make sure we're only showing supported functionality.