README.md: add Limnoria's requirements.

They have common requirements between this repository and even stock Supybot/gribble.
Merge pull request #208 from Mkaysi/patch-1

README.md: add Limnoria's requirements.
IgnoreNonVoice: Add plugin.
IgnoreNonVoice: Add config variable enableIfModerated.
IgnoreNonVoice: Also ignore invalid commands.
IgnoreNonVoice: check if message is received on channel. Closes GH-213.