[README.md] Add documentation for GEPETTO_GUI_PLUGIN_DIRS

Author: olivier-stasse

README.md

@@ -40,6 +40,15 @@ have to be available on your machine.
   - [QCustomPlot]: for the bindings of QCustomPlot in the Python interface.
     The dependency can be made mandatory with CMake variable `BUILD_PY_QCUSTOM_PLOT`.
 
+## Environment variables and plugins
+
+When launching gepetto-gui the program tries to load the plugin omniorbserver.so. In order to specify the default path of the plugins,
+the variable `GEPETTO_GUI_PLUGIN_DIRS` can be set using the following command:
+
+```sh
+export GEPETTO_GUI_PLUGIN_DIRS=${CMAKE_INSTALL_PREFIX}/lib
+```
+
 [PythonQt]: https://github.com/gepetto/pythonqt
 [QGV]: https://github.com/gepetto/qgv
 [QCustomPlot]: https://www.qcustomplot.com/