added example of vec-file

doc/user_guide/ug_traincascade.rst

@@ -154,6 +154,8 @@ The scheme of samples creation in this case is as follows. The object instances
  
 Note that for training, it does not matter how vec-files with positive samples are generated. But ``opencv_createsamples`` utility is the only one way to collect/create a vector file of positive samples, provided by OpenCV. 
 
+Example of vec-file is available here ``opencv/data/vec_files/trainingfaces_24-24.vec``. It can be used to train a face detector with the following window size: ``-w 24 -h 24``.
+
 Cascade Training
 ================
 The next step is the training of classifier. As mentioned above ``opencv_traincascade`` or ``opencv_haartraining`` may be used to train a cascade classifier, but only the newer ``opencv_traincascade`` will be described futher.