fix: port for collab server and update docs (#4569)

.env.development

@@ -4,5 +4,5 @@ REACT_APP_BACKEND_V2_POST_URL=https://json-dev.excalidraw.com/api/v2/post/
 REACT_APP_LIBRARY_URL=https://libraries.excalidraw.com
 REACT_APP_LIBRARY_BACKEND=https://us-central1-excalidraw-room-persistence.cloudfunctions.net/libraries
 
-REACT_APP_SOCKET_SERVER_URL=http://localhost:3000
+REACT_APP_SOCKET_SERVER_URL=http://localhost:3002
 REACT_APP_FIREBASE_CONFIG='{"apiKey":"AIzaSyCMkxA60XIW8KbqMYL7edC4qT5l4qHX2h8","authDomain":"excalidraw-oss-dev.firebaseapp.com","projectId":"excalidraw-oss-dev","storageBucket":"excalidraw-oss-dev.appspot.com","messagingSenderId":"664559512677","appId":"1:664559512677:web:a385181f2928d328a7aa8c"}'

README.md

@@ -118,6 +118,10 @@ yarn start
 
 Now you can open [http://localhost:3000](http://localhost:3000) and start coding in your favorite code editor.
 
+#### Collaboration
+
+For collaboration, you will need to set up [collab server](https://github.com/excalidraw/excalidraw-room) in local.
+
 #### Commands
 
 | Command            | Description                       |