Create better README, update wikis

[rennergade]

There's basically no README and the wikis may be updated here.

@kpan2034 can you update these as you learn about RustPOSIX?

[naveensrinivasan]

I would like to understand this project better and would appreciate some docs. cc @JustinCappos

[JustinCappos]

I'd love to know your thoughts on the docs when you have a chance to look over them. I know this is a process, so feel free to give direct feedback as to what else isn't clear...