- The README is better targetted to newcomers and explains the basics of
  the project: what it is, why use it, how to get started, where to find
  more information. It looks less like a reference and more like a brief
  overview of rebar3.
- Whatever looks like a list or reference manual material is expected to
  be moved to the website
- Be clearer about the rebar3 project structure in CONTRIBUTING.md;
  explain how to write tests, how to structure code, and so on.
- Added contributing section on bug reports, feature requests, etc.

Extend CONTRIBUTING.md with regards to tests and credit.

While at it, refactor the guide for clarity. Some of the new text was
taken from erlware/relx/CONTRIBUTING.md and modified as needed.