docs: updated docs to add instruction about the IAM role

Author: theburningmonk

README.md

@@ -315,8 +315,6 @@ stepFunctions:
       definition:
 ```
 
-#### HTTP Endpoint with Extended Options
-
 Here You can define an POST endpoint for the path posts/create.
 
 ```yml
@@ -330,6 +328,22 @@ stepFunctions:
       definition:
 ```
 
+#### 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:
+
+```yml
+stepFunctions:
+  stateMachines:
+    hello:
+      events:
+        - http:
+            path: posts/create
+            method: POST
+            iamRole: arn:aws:iam::<accountId>:role/<roleName>
+      definition:
+```
+
 #### Share API Gateway and API Resources
 
 You can [share the same API Gateway](https://serverless.com/framework/docs/providers/aws/events/apigateway/#share-api-gateway-and-api-resources) between multiple projects by referencing its REST API ID and Root Resource ID in serverless.yml as follows: