Add contribution guidelines

Author: stof

.github/CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# How to contribute
+
+## Generated code
+
+Most code in this package is generated by the
+[async-aws code generator](https://packagist.org/packages/async-aws/code-generator).
+
+Changes should be done in the manifest file and code should be regenerated
+instead of changing it directly, as manual changes will be lost when the code
+is regenerated. The CI will enforce that rule.
+To regenerate the code, run:
+
+```bash
+./generate --all
+```
+
+As Amazon Incentives does not provide an API description for SDKs in the format
+used for AWS SDKs, we are maintaining it ourselves in the
+[`manifest`](../manifest/) folder. This manifest is partial. It only covers the
+endpoints supported by the SDK.