Add maxAge property documentation

Author: vlewin

README.md

@@ -179,9 +179,9 @@ provider:
   name: aws
   apiGateway:
     # REST API resource ID. Default is generated by the framework
-    restApiId: xxxxxxxxxx 
+    restApiId: xxxxxxxxxx
     # Root resource, represent as / path
-    restApiRootResourceId: xxxxxxxxxx 
+    restApiRootResourceId: xxxxxxxxxx
 
 functions:
   ...
@@ -197,7 +197,7 @@ provider:
     restApiId: xxxxxxxxxx
     restApiRootResourceId: xxxxxxxxxx
     # List of existing resources that were created in the REST API. This is required or the stack will be conflicted
-    restApiResources: 
+    restApiResources:
       /users: xxxxxxxxxx
 
 functions:
@@ -257,6 +257,21 @@ stepFunctions:
 
 Configuring the cors property sets Access-Control-Allow-Origin, Access-Control-Allow-Headers, Access-Control-Allow-Methods,Access-Control-Allow-Credentials headers in the CORS preflight response.
 
+To enable the Access-Control-Max-Age preflight response header, set the maxAge property in the cors object:
+
+```yml
+stepFunctions:
+  stateMachines:
+    SfnApiGateway:
+      events:
+        - http:
+            path: /playground/start
+            method: post
+            cors:
+              origin: '*'
+              maxAge: 86400
+```
+
 #### Customizing request body mapping templates
 
 The plugin generates default body mapping templates for `application/json` and `application/x-www-form-urlencoded` content types. If you'd like to add more content types or customize the default ones, you can do so by including them in `serverless.yml`: