MAINTAINING.md: Improve review process between multiple maintainers

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

MAINTAINING.md

@@ -119,14 +119,25 @@ complete that no input from the original developer or maintainer is required.
 
 ### Organisation of reviewing between maintainers
 
-This section is a placeholder for further discussion around making the review
+#### Partial review
-process more efficient and effective when multiple maintainers are involved.
-This could include:
 
--   Usage of labels
+You can review a PR partially. This would involve reviewing all points in one or
--   Division of review responsibilities, e.g. with ACKs for different areas of
+more sections outlined in the [technical guidelines](#technical-guidelines).
-    review
+In that case, please do not "approve" the PR to prevent accidental merges.
--   Usage of GitHub functionality, for example "Reviewers" and "Assignees" lists
+Rather, give your verbal ACK and describe what you reviewed. In addition, if you
+processed or reasonably stepped over a whole section, mark the PR with the
+according label from the "Reviewed:" category. If you set a label by stepping
+over a section, please articulate your reasoning for this clearly, as noted in
+the [introduction](#introduction). This will help other maintainers help to
+better understand your line of thought. If you disagree with the assessment of
+a previous review, you may remove a certain "Reviewed:" label. Please state your
+reasoning in this case as well.
+
+When all "Reviewed:" labels are set you may give your approval for the PR.
+
+As everything in this document this is a "CAN", not a "MUST": It might help
+other maintainers to track your work, but if the overhead isn't justified, a
+simple approving ACK might suffice.
 
 #### Github etiquette