extend profiler article about how to use the stopwatch

profiler/stopwatch.rst

@@ -0,0 +1,17 @@
+.. index::
+   single: Profiling; Stopwatch
+
+How to show custom timing
+=========================
+
+Inject :class:`Symfony\\Component\\Stopwatch\\Stopwatch` by autowiring. Or do ``$stopwatch = $this->get('debug.stopwatch');`` in a Controller.
+
+All timing mesurements done by this stopwatch are shown in the profiler on the page performance.
+
+short example::
+
+    $stopwatch->start('anEvent', 'customCategory');
+    $stopwatch->lapse('anEvent');
+    $stopwatch->stop('anEvent');
+
+:doc:`/component/stopwatch` explains the methods in more details.