Add a doc example

doc/library/sandbox/cuda/dnn.txt

@@ -24,6 +24,13 @@ There are at least three possible ways of doing so:
   ``LD_LIBRARY_PATH``, ``LIBRARY_PATH`` and ``CPATH`` to the directory
   extracted from the download. If needed, separate multiple directories
   with ``:`` as in the ``PATH`` environment variable.
+
+  example::
+
+      export LD_LIBRARY_PATH=/home/user/path_to_CUDNN_folder/lib64:$LD_LIBRARY_PATH
+      export CPATH=/home/user/path_to_CUDNN_folder/include:$CPATH
+      export LIBRARY_PATH=/home/user/path_to_CUDNN_folder/lib64:$LD_LIBRARY_PATH
+
 - And as a third way, also on Linux, you can copy the ``*.h`` files
   to ``/usr/include`` and the ``*.so*`` files to ``/lib64``.