message: Add note about pkg-config usage in tutorial
diff: @@ -101,6 +101,7 @@ Building OpenCV from Source Using CMake
  - Add this flag when running CMake: `-DOPENCV_GENERATE_PKGCONFIG=ON`
  - Will generate the .pc file for pkg-config and install it.
  - Useful if not using CMake in projects that use OpenCV
  + - Installed as `opencv4`, usage: `pkg-config --cflags --libs opencv4`
  -# Build. From build directory execute *make*, it is recommended to do this in several threads
  