docs(contributing): update codegen guidance (#3535)

Author: AllanZhengYP

CONTRIBUTING.md

@@ -113,7 +113,7 @@ lerna run test --scope [package name]
 If `lerna` is not available in your environment, you can install it globally with:
 
 ```
-npm install -g lerna
+npm install -g lerna@3
 ```
 
 If you have touched multiple packages and you want to make sure the package
@@ -128,11 +128,32 @@ You don't need to run this command if the dependency packages are not changed.
 
 ### Generating Service Clients
 
-Please refer [codegen](./codegen/README.md)
+If you made changes to either [AWS Smithy TypeScript generator](./codegen/smithy-aws-typescript-codegen)
+or [Smithy TypeScript generator][smithy typescript repo], you should include
+the generated code change to your PR. Here's how to generate clients:
+
+1. Clone and checkout smithy-typescript repo
+      ```
+      git clone https://github.com/awslabs/smithy-typescript.git && cd smithy-typescript
+      git checkout [working_branch]
+      ```
+
+1. Build the local branch and publish locally:
+      ```
+      ./gradlew clean build publishToMavenLocal
+      ```
+
+1. Generate service clients in JavaScript SDK repo:
+      ```
+      yarn generate-clients
+      ```
+      `generate-clients` is a util script to facilitate the code generation. For more
+      information, please refer [codegen](./codegen/README.md)
 
 [issues]: https://github.com/aws/aws-sdk-js-v3/issues
 [pr]: https://github.com/aws/aws-sdk-js-v3/pulls
 [license]: http://aws.amazon.com/apache2.0/
 [cla]: http://en.wikipedia.org/wiki/Contributor_License_Agreement
 [aws service models]: https://github.com/aws/aws-sdk-js-v3/tree/main/models
 [conventional commits]: https://www.conventionalcommits.org/
+[smithy typescript repo]: https://github.com/awslabs/smithy-typescript