Formalize the public feature/topic branch policy

We recently discussed a range of aspects around public feature/topic branches on IRC.

In general, I'm not a fan of too much bureaucracy/overhead, but formalizing these discussion points would probably help us to move forward on some of the bigger topics in a more controlled way.

Partially using IETF/RFC terminology for clarity.

This initial PR represents my own understanding only.  It is meant to supply a concrete rubbing surface for further discussion, so if you disagree, please consider to disagree wisely and sensibly. :heart: