explain how to debug install

tsa/howto/new-machine-hetzner-robot.mdwn

@@ -76,6 +76,13 @@ The last two are passed to `grml-debootstrap` and should rarely be
 changed (although they could be coverted in to Fabric tasks
 themselves).
 
+If a problem occurs in the install, you can login to the rescue shell
+with:
+
+    ssh -o FingerprintHash=md5 -o UserKnownHostsFile=~/.ssh/authorized_keys.hetzner-rescue root@88.99.194.57
+
+... and check the fingerprint against the email provided by Hetzner.
+
 See [[new-machine]] for post-install configuration steps, then follow
 [[new-machine-mandos]] for setting up the mandos client on this host.