Explain `namespace` aliases in README

I was confused by the `resource` method used in the very first example in the README, because it was not mentioned anywhere else. Searching the source showed it was just an alias of `namespace`.

It looks like this has confused other people as well:
https://groups.google.com/forum/?fromgroups=#!topic/ruby-grape/H4BU573Lb9E

This PR adds some very brief explanation.