Readme: Mention that e.g. first-of-type is a "naked symbol"

It took me a while to understand how to use the predef. selector types such as first-of-type.
(I have tried :first-of-type, (first-of-type), and searched for namespace to refer it from.)
It would be, I suppose, helpful to state explicitely that it is used just as a symbol.
Merge pull request #17 from jakubholynet/patch-3

Readme: Mention that e.g. first-of-type is a "naked symbol"