Update CONTRIBUTING.md with guidelines for commits, PRs, reviews and merges

The `CONTRIBUTING.md` has the following:
```
Before your contribution can be accepted by the project team contributors must
electronically sign the Eclipse Contributor Agreement (ECA).

* http://www.eclipse.org/legal/ECA.php

Commits that are provided by non-committers must have a Signed-off-by field in
the footer indicating that the author is aware of the terms by which the
contribution has been provided to the project. The non-committer must
additionally have an Eclipse Foundation account and must have a signed Eclipse
Contributor Agreement (ECA) on file.
```

I think we need to consider having a lot more in there:

Code
====
+ code should use https://github.com/eclipse-ee4j/ee4j code style
+ Do we want to run check style also?

Pull Requests
==========
+ Should PRs from committers use branches within the repo or come from personal forks?

Merging
======
+ CI must pass
+ Must be reviewed by at least 1 project leader?
+ Should we have a minimum age so that committers in all timezones PRs have a chance to review?
+ Should any committer be able to call for a formal `-1`, `0`, `+1` style vote on a PRs (as is described in the [EDP](https://www.eclipse.org/projects/dev_process/#4_7_Committers_and_Contributors))?
+ With votes, who is it that may adjudicate if a `-1` is an `Invalid Veto` as per the process?

I think we need to discuss what guidelines we want, formulate them into a PR on `CONTRIBUTING.md` and then have an EDP style vote on it to make it official.

Start with CONTRIBUTING.md and compare its current ECA and Signed-off-by guidance with the proposed sections for code, pull requests, reviews and merges. The issue calls for discussing and formulating the desired rules into a CONTRIBUTING.md pull request, followed by an EDP-style vote; completion depends on those guidelines being agreed and made official.

Written by the indexing model from the issue text.