Added empty payload note to remove() docs

Not knowing this particular information caused a coworker and I to spend
4 or 5 hours trying to figure out why Chrome was refusing to make the
remove request.  Long story short, the issue I linked to
(https://github.com/mgonto/restangular/issues/193) helped solve the
problem w/o using the $http workaround
Merge pull request #781 from kjlubick/master

Added empty payload note to remove() docs