Add information about using arbitrary widths

Since this key part of how sass-enhance wasn't yet mentioned in the
documentation, it didn't feel complete.

Change-Id: I4853f5ce9ec2f1c7e127b989c77b664e062e4b81
Reviewed-on: https://gerrit.causes.com/31490
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>