Add features.override.yaml to .gitignore and update README with override instructions

Author: Ostap-Zherebetskyi

.gitignore

@@ -122,6 +122,7 @@ website/static/sitemaps/
 ##############
 
 api/static/vendor
+osf/features.override.yaml
 
 # Local settings files
 local.py

README-docker-compose.md

@@ -294,6 +294,11 @@
       ```bash
       docker compose run --rm web python3 manage.py reset_db --noinput
       ```
+- OPTIONAL: Create a `features.yaml` override
+  ```bash
+  cp osf/features.yaml osf/features.override.yaml
+  ```
+  Note: Update the WAFFLE_VALUES_YAML variable to point to the override file (osf/features.override.yaml).
 
 ## Application Debugging