book: Add another paragraph

book/src/todo_2.md

@@ -115,7 +115,10 @@ Filename: <a class=file-link href="https://github.com/gtk-rs/gtk4-rs/blob/master
 ```
 
 We also add an action which allows us to remove done tasks. 
-<!-- TODO -->
+This time we use another method called [`install_action`](https://gtk-rs.org/gtk4-rs/stable/latest/docs/gtk4/subclass/widget/trait.WidgetClassExt.html#method.install_action).
+This method has a couple of limitation.
+It can only be used when subclassing widgets, and it doesn't support stateful actions.
+On the flipside, its usage is concise and it has a corresponding sister-method [`install_action_async`](https://gtk-rs.org/gtk4-rs/stable/latest/docs/gtk4/subclass/widget/trait.WidgetClassExt.html#method.install_action_async) which we will use in one of the future chapters.
 
 Filename: <a class=file-link href="https://github.com/gtk-rs/gtk4-rs/blob/master/book/listings/todo/2/window/imp.rs">listings/todo/2/window/imp.rs</a>