Mention how to use custom nameservers.

Explain that looking up username@example.com will use the
example.com nameserver.

GitLab: jami-project#811
Change-Id: I678fdf508dc555fd0c6e16ff0da48d0df478d2ff

developer/name-server-protocol.md

@@ -11,6 +11,11 @@ The public nameserver is hosted at `ns.jami.net` and uses a blockchain
 as its backend.  Another implementation could use any other database
 or directory service making the nameserver protocol reusable.
 
+If you run your own nameserver, looking up a username in the form of
+`username@example.com` will look up the name `username` with the
+nameserver at `example.com`.  (There is no need to add `@ns.jami.net`
+to use the default nameserver.)
+
 Rules on name formatting
 ------------------------
 

user/faq.rst

@@ -281,6 +281,9 @@ one at ``ns.jami.net``.
 If you don't register a username, you can still choose to register one
 later at any time.
 
+If you host your own nameserver at ``example.com``, usernames registered
+there can be looked up by searching for ``username@example.com``.
+
 Can I change my username?
 ~~~~~~~~~~~~~~~~~~~~~~~~~