I was surprised that nobody mentioned the Collective Code Construction Contract of zeromq/Pieter Hintjens [1]. It tries to minimise the friction created by maintaining & contributing to open source projects.
It is not perfect of course, but at least it is a good start. Especially the "value-" & opinion-based discussions can be reduced considerably.
If you manage an open source project, you owe it to yourself to make it clear how other devs can contribute and add features they want to see. Don’t be the choke point. Open it up. That’s the whole point. Make it clear how a user can add to the codebase and encourage them to do so.