docs: introduce action option for event.http to README.md

Author: shota

README.md

@@ -17,6 +17,7 @@ This is the Serverless Framework plugin for AWS Step Functions.
  - [Events](#events)
      - [API Gateway](#api-gateway)
          - [Simple HTTP endpoint](#simple-http-endpoint)
+         - [Custom Step Functions Action](#custom-step-functions-action)
          - [HTTP Endpoint with custom IAM Role](#http-endpoint-with-custom-iam-role)
          - [Share API Gateway and API Resources](#share-api-gateway-and-api-resources)
          - [Enabling CORS](#enabling-cors)
@@ -375,6 +376,40 @@ stepFunctions:
       definition:
 ```
 
+#### Custom Step Functions Action
+
+Step Functions have custom actions like DescribeExecution or StopExecution to fetch and control them. You can use custom actions like this:
+
+```yml
+stepFunctions:
+  stateMachines:
+    start:
+      events:
+        - http:
+            path: action/start
+            method: POST
+      definition:
+        ...
+    status:
+      events:
+        - http:
+            path: action/status
+            method: POST
+            action: DescribeExecution
+      definition:
+        ...
+    stop:
+      events:
+        - http:
+            path: action/stop
+            method: POST
+            action: StopExecution
+      definition:
+        ...
+```
+
+Request template is not used when action is set because there're a bunch of actions. However if you want to use request template you can use [Customizing request body mapping templates](#customizing-request-body-mapping-templates).
+
 #### HTTP Endpoint with custom IAM Role
 
 The plugin would generate an IAM Role for you by default. However, if you wish to use an IAM role that you have provisioned separately, then you can override the IAM Role like this: