Update tuns documentation

Author: antoniomika

posts/tuns.md

@@ -127,6 +127,15 @@ Once set up, you can then create tunnels via your custom domain like this:
 ssh -R customdomain.example.com:80:localhost:8000 tuns.sh
 ```
 
+> NOTICE: Make sure to select the correct (closest) tuns instance. You can find the instance
+> you're connected to from the connection output from tuns:
+> "You are connected to tuns.sh. The following tunnels are only accessible on this instance."
+> In this case, my CNAME would use tuns.sh
+>
+> You may want to pre-select the region you connect to. Try pinging ash.tuns.sh or nue.tuns.sh
+> to find the instance closest to you (lowest latency), and use that for both your SSH connection
+> and CNAME.
+
 # tunmgr
 
 A tunnel manager for docker services.