document how to rejoin the AD

admin-guide/troubleshooting/deployment.rst

@@ -89,3 +89,11 @@ reasons:
   /etc/puppetlabs -name '*.pem' -delete`` and clean up any certificates on the
   Puppet server with ``puppet cert clean $HOSTNAME``. Then restart the
   installation.
+
+Rejoin the Active Directory
+===========================
+
+If the AD join seams to be broken (failed logins, etc.), then the node can be automatically rejoined again:
+
+- remove ``/etc/krb5.keytab``
+- run puppet, e.g. with ``puppet agent --test``