Add related read

Author: compulim

docs/SORTING.md

@@ -120,6 +120,8 @@ Combining the algorithms above, Web Chat has the following assumptions
 
 ## Related read
 
+-  Pull requests
+   - [Grouped activities are sorted as a single unit](https://github.com/microsoft/BotFramework-WebChat/pull/5635)
 -  Posting an activity, [`sagas/postActivitySaga.ts`](https://github.com/microsoft/BotFramework-WebChat/blob/main/packages/core/src/sagas/postActivitySaga.ts)
 -  Inserting an activity into the chat history, [`reducers/activities.ts`](https://github.com/microsoft/BotFramework-WebChat/blob/main/packages/core/src/reducers/activities.ts)
 -  Activity type, [`types/WebChatActivity.ts`](https://github.com/microsoft/BotFramework-WebChat/blob/main/packages/core/src/types/WebChatActivity.ts)