Added documentation of new config

doc/library/config.txt

@@ -174,6 +174,14 @@ import theano and print the config variable, as in:
 
     Print active device at when the GPU device is initialized.
 
+.. attribute:: enable_initial_driver_test
+
+    Bool value: either ``True`` or ``False``
+
+    Default: ``True``
+
+    Tests the nvidia driver when a GPU device is initialized.
+
 .. attribute:: floatX
 
     String value: either 'float64' or 'float32'