doc/CONTRIBUTION.md: document mediation process

[maribu]

Contribution description

As agreed upon in the weekly coordination meeting we want to set up a channel / tool / process to aid contributors to solve conflicts.

This is especially aimed to empower contributors to solve conflicts with maintainers, as we were unsure that the existing code of conduct violation mechanism is sufficient to address conflicts that do not involve a CoC violation.

Testing procedure

• Wait for the infrastructure to contact mediators to be in place and check if the address matches
• Check if the wording is clear
• Check if the doc is easy to find
• Check if it is made clear that the mechanism is intended to be a constructive tool aimed at solving the conflict
  • specifically: it should be clear to all parties than invoking the mediation should be seen as "blame game"

Issues/PRs references

None

[mguetschow]

Thanks a lot for putting effort into this! LGTM on a first read, just one typo and a suggestion below.