Be decent to each other. That is the whole rule, and in a project this size it has always been enough.
In practice: assume good faith, keep criticism about the code, and remember that everyone here is doing this in their spare time. Disagreement is fine and useful, contempt is not.
If someone is making the project worse to be part of, mail manager@perfecto-web.com. Reports stay private. The maintainer can edit or remove comments, close issues and pull requests, and block accounts.