doc: document STARPU_IDLE_FILE

doc/doxygen/chapters/501_environment_variables.doxy

@@ -1,7 +1,7 @@
 /* StarPU --- Runtime system for heterogeneous multicore architectures.
  *
  * Copyright (C) 2011-2013,2015-2017                      Inria
- * Copyright (C) 2010-2017                                CNRS
+ * Copyright (C) 2010-2018                                CNRS
  * Copyright (C) 2009-2011,2013-2017                      Université de Bordeaux
  * Copyright (C) 2016                                     Uppsala University
  *
@@ -1173,6 +1173,14 @@ If StarPU doesn't find any NUMA node after these step, STARPU_MAIN_MEMORY is the
 discovered by StarPU.
 </dd>
 
+<dt>STARPU_IDLE_FILE</dt>
+<dd>
+\anchor STARPU_IDLE_FILE
+\addindex __env__STARPU_IDLE_FILE
+If the environment variable STARPU_IDLE_FILE is defined, a file named after its contents will be created at the end of the execution.
+The file will contain the sum of the idle times of all the workers.
+</dd>
+
 </dl>
 
 \section ConfiguringTheHypervisor Configuring The Hypervisor