Explain how to run the manual test

Adds a comment about the preconditions to execute the test.

Change-Id: Iafb56ec18520e02681cce73e406f0e9744662a77
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/manual/websockets/tst_websockets.cpp

@@ -32,6 +32,12 @@
 #include <QDebug>
 #include "QtWebSockets/QWebSocket"
 
+// Run 'wstest -m echoserver -w ws://localhost:9000' before launching
+// the test.
+
+// wstest is part of the autobahn-testsuite:
+// https://github.com/crossbario/autobahn-testsuite
+
 class tst_WebSocketsTest : public QObject
 {
     Q_OBJECT