end2end-tests: update metadata.scale + documentation to do it

resources/README.md

 # Resources
 
 Files used for different purpose like tests.
+
+## metadata.scale
+
+To generate it for GDev:
+
+    cargo run -- --dev --tmp
+    subxt metadata -f bytes > resources/metadata.scale
+
+See [Upgrade metadata](../end2end-tests/README.md#upgrade-metadata) for more details.