Contributing document could use improvement

[mithro]

The CONTRIBUTING.md file in the github/docs includes a lot of the information we should include.

At a minimum we should explain;

• We use a "fork and pull request" model of development. Potential things we should link to are (top links on Google Search);
  • https://gist.github.com/Chaser324/ce0505fbed06b947d962
  • https://reflectoring.io/github-fork-and-pull/
• Both maintainers and external contributors are expected to follow the same set of processes.

It should still include information about the CLA process.

[mithro]

We should also add something around proprietary tooling support policy.