Document how to change the listening port

debian/tang.README.Debian

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+Changing default listening port
+===============================
+
+The default listening port for the tangd service is 80, the usual HTTP
+port. If this causes a collision with other services, use
+
+    systemctl edit tangd.socket
+
+to create an override.
+
+To activate any changes:
+
+    systemctl restart tangd.socket
+
+As an example, the following content will
+
+* disable listening on port 80
+* enable listening on port 81 instead
+
+    [Socket]
+    ListenStream=
+    ListenStream=81
+
+See systemd.socket(5) for more options.