They start with good intentions (informing the team about what's expected of them and how to work efficiently with the team) but they usually devolve into justifying the manager's bad behaviors and telling the team to deal with it.
In practice, most manager READMEs I've seen have been more aspirational than realistic. That is, managers describing themselves in the best possible way, or describing the type of manager they want to be.
I think it's much better to write an "Employee how-to manual" that describes what's expected of the employees, how employees can accomplish things, and so on. Delivering information to the employees from a purely manager-centric document sends a message that the manager is the star of the show and the employees are just the supporting cast who need to deal with the manager's quirks.