Clarify choice of username at registration, again

Apparently it was still unclear to people what call they should use on
registration. Added extra message to explain this further.

templates/registration/register.html

@@ -10,8 +10,11 @@
 			<div class="panel-body">
 				<p>
 				Please register with your (uppercase) Callsign as Usernames.
-				For DN-Calls, -[0-9] is allowed.
+				For DN-Calls, -[0-9] is allowed (e.g. DN1ABC-2 for the second group).
 				</p>
+                <p>
+                Note: If you are a <strong>Ham/OM/YL</strong> please with your <strong>own</strong> callsign (e.g. DL7DOC). If you are a <strong>SWL</strong>, please use the <strong>DN-Call provided</strong> by your operator.
+                </p>
 				<form method="POST" action="{% url 'register' %}">
 				{% csrf_token %}
 				{{ form|crispy }}