Document the contribution process

While I have not participated much to the project, I have been around for a while. It was never particularly clear:
- who has meaningful(?) review powers
- who can merge MRs - let's call those committers
- how can one become a committer
- what is the start-to-end process for having a PR merged (and ultimately released)

In particular - one can deduce from "git log" individuals who have merged MRs in the past, similarly looking at the github UI one can see people who have approved a PR. At the same time there are multiple PRs being approved by 2 committers while the PR is still open, in some cases for over 2 months. Note that I'm talking about PRs with no objections or pending requests.

Similarly we have PRs reviewed by third parties, where committers have neither reviewed nor merged - in some cases for well over 6 months.

Can we please have an outline in CONTRIBUTING.md (or elsewhere) for the above bullet-points? It's much easier if people know what's the process they should follow - are third parties allow to review or comment or PRs, who has merge access, how to gain merge access, etc.

@XinfengZhang @xhaihao @uartie @dvrogozh tagging some Intel folks who have significant contributions - hopefully someone can shed some light and propose a documentation.