Policy on project policies

[dhardy]

@pitdicker this is especially aimed at you as a core contributor but others are very welcome to comment.

Currently the policies document is a wiki article which makes it easy to update but hard to notify others of changes.

For the record: I just added a section on use of unsafe.

Suggestion: should we add a document to the main repo, e.g. doc/policies.md, which we only update via PRs?

Are there more important policies we should add here? Do we need a separate contributor guide? However, lets not add unnecessary documentation.

[dhardy]

Implemented: #544