adding in TmuxpPlugin.__init__ documentation to the online docs

Author: joseph-flinn

docs/plugin_system.rst

@@ -123,8 +123,9 @@ in a configuration file like the following:
 Plugin API
 ----------
 
+.. automethod:: tmuxp.plugin.TmuxpPlugin.__init__
 .. automethod:: tmuxp.plugin.TmuxpPlugin.before_workspace_builder
 .. automethod:: tmuxp.plugin.TmuxpPlugin.on_window_create
 .. automethod:: tmuxp.plugin.TmuxpPlugin.after_window_finished
 .. automethod:: tmuxp.plugin.TmuxpPlugin.before_script
-.. automethod:: tmuxp.plugin.TmuxpPlugin.reattach
+.. automethod:: tmuxp.plugin.TmuxpPlugin.reattach