Document how to use rpath/LD_LIBRARY_PATH with GDNative

first, my english may not good enough so be patient plz
problem:
my flow tutorial: [GDNative C++ example](https://docs.godotengine.org/en/3.1/tutorials/plugins/gdnative/gdnative-cpp-example.html)
i try make c++ plugin which include opencv. Everything work find, i can compile libgdexample.so.
my opencv shared lib located at: /opt/opencv/lib
now is crazy time:
1. try plugin without any opencv code = > work find
2. set tiny code: cv::Mat test; in init method => /libgdexample.so. Error: libopencv_imgcodecs.so.4.1: cannot open shared object file: No such file or directory
3. export RPATH=path/to/opencvlib, re-try again => still cant find shared lib opencv
4. turn of godot (my godot run from steam), re-open by call executable from terminal => it work
5. export project to executable file => it still require run from terminal, if not, library not found again
All my work on ubuntu 18.04 and godot 3.1

any one know how to run executable directly (double click on run file) ?