cmd/derper: clarify that derper and tailscaled need to be in sync

Fixes #12617

Change-Id: Ifc87b7d9cf699635087afb57febd01fb9a6d11b7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f