Fixes #212 - adds contributing rules to readme.
Fixes #212 - adds contributing rules to readme.

Adds more information on how to contribute and where to put tests.
GerHobbelt/clean-css,HossainKhademian/CleanCss,scottgonzalez/clean-css,yomed/clean-css,island205/clean-css,vlajos/clean-css,jakubpawlowicz/clean-css,tomByrer/clean-css,webconnex/clean-css,stuht/clean-css,MartyIX/clean-css,shelsonjava/clean-css