Adds missing entry to Readme about broken module compatibility.
Adds missing entry to Readme about broken module compatibility.

* see 3f1e064c91474ff9bd45ea39b11bc7bd564125c6.

yomed/clean-css,HossainKhademian/CleanCss,vlajos/clean-css,island205/clean-css,webconnex/clean-css,stuht/clean-css,GerHobbelt/clean-css,jakubpawlowicz/clean-css,scottgonzalez/clean-css,MartyIX/clean-css,shelsonjava/clean-css