Improve docs for sharing logic in parent and child features.

Sources/ComposableArchitecture/Documentation.docc/Articles/Performance.md

@@ -382,6 +382,28 @@ store.send(.textFieldChanged("Hello") {
 }
 ```
 
+##### Sharing logic in child features
+
+There is another common scenario for sharing logic in features where the parent feature wants to
+invoke logic in a child feature. One can technically do this by sending actions from the parent 
+to the child, but we do not recommend it (see above in <doc:Performance#Sharing-logic-with-actions>
+to learn why):
+
+```swift
+// Handling action from parent feature:
+case .buttonTapped:
+  // Send action to child to perform logic:
+  return .send(.child(.refresh))
+```
+
+Instead, we recommend invoking the child reducer directly:
+
+```swift
+case .buttonTapped:
+  return Child().reduce(into: &state.child, action: .refresh)
+    .map(Action.child))
+```
+
 ### CPU intensive calculations
 
 Reducers are run on the main thread and so they are not appropriate for performing intense CPU