To learn more about the plugin system see documentation of reForis.
By default, the latest version of reForis package is installed automatically when executing make prepare-dev command. If you wish to use development (editable) version from a local directory run make install-local-reforis. This assumes that reForis is in reforis directory alongside plugin directory.
Please note that you still need to upload reForis code to the router for changes to be taken into account.