Add documentation about yaml file used to add a simple way to add / delete plugin

With https://gitlab.fusiondirectory.org/fusiondirectory/fd/-/issues/6196 we must provided a correct definition a control.yaml file Current definition could be used :

• see : https://github.com/gallak/fusiondirectory-plugins-pluginsmanager/blob/main/README.md
• An example : https://github.com/gallak/fusiondirectory-plugins-pluginsmanager/blob/main/control.yaml