Add explanatory code comment

Author: pglombardo

INSTALLATION.md

@@ -131,6 +131,7 @@ if __name__ == "__main__":
     application = tornado.web.Application([
         (r"/", MainHandler),
     ])
+    # Wrap the Tornado WSGI application with the Instana WSGI Middleware (iWSGIMiddleware)
     wsgi_app = iWSGIMiddleware(tornado.wsgi.WSGIAdapter(application))
     server = wsgiref.simple_server.make_server('', 8888, wsgi_app)
     server.serve_forever()