Add documentation for the rb-extension-pack repository.
People were a bit confused about the purpose of rb-extension-pack and it

was unclear which extensions were usable and which weren't. To fix this,

I'm adding a series of READMEs, plus a CONTRIBUTING file for GitHub, to

make it clear what the status and requirements are for each extension.
Going forward, every extension should provide a README in the same

format.

Went through the docs. Not much to test.