Add note about updating your signing keys (ie. "the auto thing")

README.rst

@@ -128,6 +128,15 @@ To set up your homeserver, run (in your virtualenv, as before)::
 
 Substituting your host and domain name as appropriate.
 
+This will generate you a config file that you can then customise, but it will
+also generate a set of keys for you. These keys will allow your Home Server to
+identify itself to other Home Servers, so don't lose or delete them. It would be
+wise to back them up somewhere safe. If, for whatever reason, you do need to
+change your Home Server's keys, you may find that other Home Servers have the
+old key cached. If you update the signing key, you should change the name of the
+key in the <server name>.signing.key file (the second word, which by default is
+, 'auto') to something different.
+
 By default, registration of new users is disabled. You can either enable
 registration in the config by specifying ``enable_registration: true``
 (it is then recommended to also set up CAPTCHA), or