There's a great blog post that I found by Camille Fournier that touches on things that you've mentioned https://medium.com/@skamille/i-hate-manager-readmes-20a0dd9a..., but I think something of note is at the end on how it can be used as a self-exercise for the manager.

tl;dr, Manager README's are more for the manager than the direct report. It's a great exercise in introspection if kept private and not used as something codified for employees to follow.