Help Wanted: Documentation improvements

[wbthomason]

packer can do a lot, and has a lot of knobs to tune, but the current docs (README and vimdocs) do a poor job of surfacing all of the possibilities in a clear and easy to understand way. Additionally, there's currently no FAQ/similar, which (as mentioned in #134 and elsewhere) would make packer easier to use.

If anyone is willing to help write better docs, I would be highly appreciative and happy to provide guidance!