add_rapp_function() adds menu option on the right side, while add_lapp_function on left of the menu.
_init/config file is just text file in format key:value containing various informations about module package. Some fields can have mutliline content (e.g. Description) - in this case continuation lines begin from white space in first column.
If you want to share some extension module via our repository adding any config file with basic information (author, www, name, description) leaving rest just empty is enough. We will fix it if necessary. On the other hand if you want to use the module only locally the _init/config file is not needed.