docs: Add documentation for AWS_LWA_ERROR_STATUS_CODES environment variable

Author: bnusunny

README.md

@@ -138,6 +138,8 @@ Please check out [FastAPI with Response Streaming](examples/fastapi-response-str
 
 **AWS_LWA_AUTHORIZATION_SOURCE** - When set, Lambda Web Adapter replaces the specified header name to `Authorization` before proxying a request. This is useful when you use Lambda function URL with [IAM auth type](https://docs.aws.amazon.com/lambda/latest/dg/urls-auth.html), which reserves Authorization header for IAM authentication, but you want to still use Authorization header for your backend apps. This feature is disabled by default.
 
+**AWS_LWA_ERROR_STATUS_CODES** - A comma-separated list of HTTP status codes that will cause Lambda invocations to fail. Supports individual codes and ranges (e.g. "500,502-504,422"). When the web application returns any of these status codes, the Lambda invocation will fail and trigger error handling behaviors like retries or DLQ processing. This is useful for treating certain HTTP errors as Lambda execution failures. This feature is disabled by default.
+
 ## Request Context
 
 **Request Context** is metadata API Gateway sends to Lambda for a request. It usually contains requestId, requestTime, apiId, identity, and authorizer. Identity and authorizer are useful to get client identity for authorization. API Gateway Developer Guide contains more details [here](https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-input-format).