Document "exclude" option, implemented in #23

Just a simple documentation change.  I didn't know that `exclude` was an option (implemented in #23, and still present).  I know [I'm not the only one](http://stackoverflow.com/questions/9445988/is-there-a-way-to-configure-guard-to-not-run-request-specs) who has wanted this, so it's worth documenting.  :smile: