Improve documentation

[campoy]

This library is used by go-git and as such many developers get to use it.

Currently the documentation is quite poor. See https://godoc.org/github.com/src-d/go-billy, there's zero package level documentation explaining what this library is for.

It'd be also nice to have a comparison with https://github.com/spf13/afero.

It is also missing a CONTRIBUTING.md and references to Code of Conduct etc as requested by https://github.com/src-d/guide/blob/master/engineering/documentation.md

[MMulthaupt]

To throw in some specifics, how to get a sensible default implementation of Filesystem would be nice to know.