During RIOT Summit 2018 the attending maintainers found that the
organization for the review process involving multiple maintainers is
lacking both in process description and existing mechanisms.

This changes the "Organisation of reviewing between maintainers"
section of the maintenance guidelines, to be in accordance with the
results of the discussions spawned from this fact.

If approved the following labels are added to GitHub:

- Reviewed: 1-fundamentals
- Reviewed: 2-code-design
- Reviewed: 3-testing
- Reviewed: 4-coding-conventions
- Reviewed: 5-documentation

- Two ACKs should be required to ensure that the maintainer guidelines
  remain credible.

- This document is the result of original work by @haukepetersen and
  refinement by @miri64, @aabadie, @kaspar030, @jcarrano,
  @PeterKietzmann, @ZetaR60, and @jia200x.