add selector example to readme

packages/signals/src/selector.rs

@@ -87,7 +87,7 @@ pub fn selector<R: PartialEq>(mut f: impl FnMut() -> R + 'static) -> ReadOnlySig
         value: f(),
     });
     {
-        // get_effect_stack().effects.borrow_mut().pop();
+        get_effect_stack().effects.borrow_mut().pop();
     }
 
     effect.callback.value.set(Box::new(move || {