docs: add notes on how to build own DERP server
The official doc is hidden under a bunch of issues. Add a doc link here and hope it could be helpful.
# This file contains some of the official Tailscale DERP servers,
# shamelessly taken from https://github.com/tailscale/tailscale/blob/main/net/dnsfallback/dns-fallback-servers.json
#
-# If you plan to somehow use headscale, please deploy your own DERP infra
+# If you plan to somehow use headscale, please deploy your own DERP infra: https://tailscale.com/kb/1118/custom-derp-servers/
regions:
1:
regionid: 1
