Add info on get_param option

Query strings are not effective at cache-busting for some proxy servers.  They are fine to use in most cases, but if you expect your users to be behind a proxy, query strings might not cut it.
See here: http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/
Merge pull request #5 from kevindice/patch-1

Add info on get_param option