Add note about pkg-config usage in tutorial

doc/tutorials/introduction/linux_install/linux_install.markdown

@@ -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