We had an internal Google doc about this (Tailscalars:
http://go/clientmod) but that doesn't help open source contributors or
agents.
So move the docs to git.
Updates #12614
Change-Id: I0b0e9f0286b23b4fb1b51ff3d41eba75edf62cdf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>