Update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,18 @@
-For changes to files under the `/codegen/aws-models` folder, and manual edits to autogenerated code (e.g. `/service/s3/api.go`) please create an Issue instead of a PR for those type of changes.
+# **PLEASE READ BEFORE CONTINUING**
+
+Do not submit pull requests that directly modify generated source files, e.g. `/service/s3/api_client.go`. Generated source files will always include an identifying header:
+
+```
+// Code generated by smithy-go-codegen DO NOT EDIT.
+```
+
+Manual changes to these files will be overwritten by code generation that occurs as part of the daily SDK release process.
+
+Do not submit pull requests that directly modify files in the `/codegen/aws-models` folder. These are API model files, owned by each AWS service team, that are updated automatically as part of the daily SDK release process. Local changes to these files will not persist.
+
+If you believe the contents of any of these files need to be changed, please [open an issue](https://github.com/aws/aws-sdk-go-v2/issues/new/choose).
+
+#
 
 If the PR addresses an existing bug or feature, please reference it here.