docs: Add AWS_LWA_ERROR_STATUS_CODES configuration to README

Author: bnusunny

README.md

@@ -105,6 +105,7 @@ The readiness check port/path and traffic port can be configured using environme
 | AWS_LWA_INVOKE_MODE                                          | Lambda function invoke mode: "buffered" or "response_stream", default is "buffered"  | "buffered" |
 | AWS_LWA_PASS_THROUGH_PATH                                    | the path for receiving event payloads that are passed through from non-http triggers | "/events"  |
 | AWS_LWA_AUTHORIZATION_SOURCE                                 | a header name to be replaced to `Authorization` | None  |
+| AWS_LWA_ERROR_STATUS_CODES                                  | comma-separated list of HTTP status codes that will cause Lambda invocations to fail (e.g. "500,502-504,422") | None  |
 
 > **Note:**
 > We use "AWS_LWA_" prefix to namespacing all environment variables used by Lambda Web Adapter. The original ones will be supported until we reach version 1.0.