doc: Improve contribution guidelines

doc: Improve contribution guidelines

- Add some guidance about the type of information a patch author should
  provide to facilitate the review (and for future reference).

- Make a number of implicit expectations explicit:
  - Every patch must compile.
  - All CI tests must pass.

- Mention that the patch author is expected to add reviewers and explain
  how to choose them.

- Explain the patch submission rules in terms of Gerrit labels.

Also do some cosmetic changes, like adding empty lines, shuffling some
paragraphs around.

Change-Id: I6dac486684310b5a35aac7353e10fe5474a81ec5
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>