update README for FE

update README for FE

Author: lnqminh3003

shatter-web/README.md

@@ -20,6 +20,7 @@ npm run dev
 - Step 5: open the port 3000 (http://localhost:3000/) to check whether it works
 
 # Note
-Don't push changes directly to main, create your own branch and pull request to merge
+- Because you are working on frontend, you MUST run both frontend and backend locally to test, check the guide `shatter-backend/README.md` to install and run backend
+- Don't push changes directly to main, create your own branch and pull request to merge