Update project README to describe the response payload for non-http events (awslabs#346)

* Implement pass-through events

* Update README file

* Update e2e test template and the README file

* Optimize e2e test job

* Update README

* Update project README

Author: bnusunny

README.md

@@ -153,7 +153,6 @@ The [Express.js](examples/expressjs/app/src/index.js) gives a simple example. Mo
 
 Lambda Web Adapter allows developers to develop web applications locally with familiar tools and debuggers: just run the web app locally and test it. If you want to simulate Lambda Runtime environment locally, you can use AWS SAM CLI. The following command starts a local api gateway endpoint and simulate the Lambda runtime execution environment.  
 
-
 ```bash
 sam local start-api
 ```
@@ -162,7 +161,7 @@ Please note that `sam local` starts a Lambda Runtime Interface Emulator on port
 
 ## Non-HTTP Event Triggers
 
-Lambda Web Adapter supports non-http event triggers (such as SQS, EventBridge, and Bedrock Agents, etc.). The adapter will forward the event payload to the web application at the path specified by environment variable `AWS_LWA_PASS_THROUGH_PATH`. The default path is `/events`. The web application can retrieve the event payload from the request body.
+Lambda Web Adapter supports non-http event triggers (such as SQS, EventBridge, and Bedrock Agents, etc.). The adapter will forward the event payload to the web application at the path specified by environment variable `AWS_LWA_PASS_THROUGH_PATH`. The default path is `/events`. The web application can retrieve the event payload from the request body. The http response body must be a valid JSON string and will be passed back as the response payload to the Lambda service.
 
 ## Examples