update readme

Added backend installation instructions to README.

Author: philip-chen6

README.md

@@ -7,4 +7,5 @@
 3. when developing: when you save it should auto reload. console logs appear in terminal, shake your device for debugging, and if you need to clear cache use npm start -- -c.
 4. if you need to add react native libraries: do npx expo install to pick compatible versions
 
-no backend needed for now!
+# Backend:
+no backend needed for now, but cd backend and npm install