added usage doc for DebugExecutionTime

SE/se-lib/DebugExecutionTime.php

@@ -1,5 +1,14 @@
 <?php
 
+/* usage:
+	Lib::loadClass('DebugExecutionTime');
+	$dbgExecTime = new DebugExecutionTime();
+	$dbgExecTime->activate();
+	$dbgExecTime->log('start');
+	...
+	$dbgExecTime->log('end');
+	$dbgExecTime->printDebug();
+*/
 class DebugExecutionTime {
 
 	private $_log = array();