docs: test guideline before a pull request

Author: shawnhankim

CONTRIBUTING.md

@@ -31,6 +31,14 @@ To suggest a feature or enhancement, please create an issue on GitHub with the l
 ### Open a Pull Request
 
 - Fork the repo, create a branch, implement your changes, add any relevant tests, submit a PR when your changes are **tested** and ready for review.
+
+  - Unit test: run `make test` for NGINX OSS and `make test-plus` for NGINX Plus
+  - Integration/Manual test: ensure that your changes work in either your app or the following runtime
+    - [`nginx-s3-gateway`](https://github.com/nginxinc/nginx-s3-gateway)
+    - [`nginx-lambda-gateway`](https://github.com/nginx-serverless/nginx-lambda-gateway)
+
+  Make sure that you also manually test this library that is integrated with the runtime such as `nginx-s3-gateway` and `nginx-lambda-gateway`.
+  
 - Fill in [our pull request template](https://github.com/nginxinc/nginx-aws-signature/blob/main/.github/pull_request_template.md).
 
 Note: if you'd like to implement a new feature, please consider creating a [feature request issue](https://github.com/nginxinc/nginx-aws-signature/blob/main/.github/feature_request_template.md) first to start a discussion about the feature.