docs: update contributing

Author: hyochan

.vscode/settings.json

@@ -43,6 +43,7 @@
     "cSpell.words": [
         "billingclient",
         "gson",
+        "hyodotdev",
         "openiap",
         "skus"
     ]

CONTRIBUTING.md

@@ -33,6 +33,13 @@ cd openiap-google
 adb shell am start -n dev.hyo.martie/.MainActivity
 ```
 
+## Generated Types
+
+- All GraphQL models in `openiap/src/main/java/dev/hyo/openiap/Types.kt` are generated from the [`hyodotdev/openiap-gql`](https://github.com/hyodotdev/openiap-gql) repository. When you update API behavior, adjust the upstream type generator first so the Kotlin output stays in sync across platforms.
+- The canonical workflow is documented in `CONVENTION.md`. Read it before touching generated models or related helpers.
+- To refresh the generated file locally, run `./scripts/generate-types.sh`. If you need to experiment with manual edits, you can pass `--skip-download true` to reuse the current `Types.kt` while still applying the post-processing step, but remember that ad-hoc edits will not ship in published releases unless the upstream generator incorporates them.
+- For changes that require generator support, open an issue or pull request in the `openiap-gql` repository.
+
 ## Code Style
 
 - Follow the official Kotlin Coding Conventions