Merge pull request #13382 from LaurentBerger:imreadsize

* try to solve #13381

* Add note

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -171,6 +171,8 @@ Currently, the following file formats are supported:
     [Vector](http://www.gdal.org/ogr_formats.html).
 -   If EXIF information are embedded in the image file, the EXIF orientation will be taken into account
     and thus the image will be rotated accordingly except if the flag @ref IMREAD_IGNORE_ORIENTATION is passed.
+-   By default number of pixels must be less than 2^30. Limit can be set using system
+    variable OPENCV_IO_MAX_IMAGE_PIXELS
 
 @param filename Name of file to be loaded.
 @param flags Flag that can take values of cv::ImreadModes