I personally find that creating and maintaining a Manager README is of great benefit, to both managers and the people they manage. You can read my Manager README here.
Mike Crittenden makes an argument for very succinct personal/manager READMEs. While I agree with his premise that "people like writing about themselves too much ...[and]... don’t know where to stop", I think having a 50 word limit swings too hard in the other direction. For me it's about knowing your audience. With a personal README I would agree — no one wants an essay about you when (digitally) meeting you for the first time. But for a manager README I'd find 50 words to be woefully lacking when trying to understand how my new manager functions.