add warning about ping in cloud installer

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

@@ -39,6 +39,8 @@ continue with:
     prefix you find on the web interface.  Make it end in `::1`)
     TODO: merge script with the [[new-machine-hetzner-robot]]
     procedure.
+    WARNING: this procedure has been known to leave `ping`
+    non-functional for regular users, see [ticket 31781](https://trac.torproject.org/projects/tor/ticket/31781)
  9. once done, note down all the info and reboot the VM: `reboot`
  10. `ssh -o FingerprintHash=sha1 root@<ipaddr>`  to unlock the host, (to compare ssh's base64 output to dropbear's b16, you can use `perl -MMIME::Base64 -e '$h = unpack("H*", decode_base64(<>)); $h =~ s/(..)(?=.)/\1:/g; print $h, "\n"'` to convert base64 to base16.
  11. `ssh root@<ipaddr>` to access it once booted