Merge pull request #20648 from maribu/doc/mediation

doc/CONTRIBUTION.md: document mediation process

CONTRIBUTING.md

@@ -9,6 +9,7 @@ of this document using the following links:
 * [Contributing code][contributing-code]
 * [Writing Documentation][writing-documentation]
 * [Working with Git][working with git]
+* [Mediate Conflicts][mediate-conflicts]
 
 If you have questions, please write a post over at our [forum] or chat on
 `#riot-os:matrix.org` on [Matrix].
@@ -363,3 +364,45 @@ PR:
 ```console
 $ git push --force-with-lease
 ```
+
+## Mediating Conflicts
+[mediate-conflicts]: #mediate-conflicts
+
+A diverse community such as the RIOT community has inherently a diverse set of
+views and opinions. Occasionally those may clash, even with all parties adhering
+to our [code of conduct][code-of-conduct]. In case you seek out help to resolve
+it you may send an email to [mediator email list][email-mediator]. The email
+will be forwarded to a panel of mediators consisting of trusted RIOT
+community members including both maintainers and non-maintainers.
+
+We see the mediation as a tool to solve conflicts, not as a way to sanction
+people. Hence, do not be afraid to contact our mediators and equally do not
+feel denounced or punished if someone invokes mediation.
+
+This is a (non-exhaustive) list of reasons you can ask for mediation:
+
+- You are a contributor and feel being treated unfairly by another contributor
+  or maintainer
+    - Your PR is rejected/blocked for unjust or non-technical reasons.
+    - A PR is rejected/blocked for valid technical reasons, but without due
+      consideration of your arguments
+    - A maintainer is asking for disproportionate changes to your PR in order
+      to get it upstream, e.g. fixing issues unrelated to your PR
+- You disagree with other contributors/maintainers on how RIOT should evolve
+    - E.g. there are competing pull requests that cover highly related use
+      cases and there is a disagreement which should be preferred
+    - A pull requests results (or resulted) deliberately in a regression for a
+      particular use case, e.g. by doing a different trade-off that benefits
+      another use case
+    - A feature relevant for your use case has been deprecated/removed
+      (e.g. to reduce maintenance burden, reduce (perceived) duplication of
+      features, etc.) but your use case relies on the feature or implementation
+      details of the feature
+
+Note: Violations of our [code of conduct][code-of-conduct] should be reported
+      to [email-coc]. See our [code of conduct][code-of-conduct] for more
+      details.
+
+[code-of-conduct]: CODE_OF_CONDUCT.md
+[email-mediator]: mailto:mediation@riot-os.org
+[email-coc]: mailto:conduct@riot-os.org