docs: added info about x-ray tracing

Author: hassaku63

README.md

@@ -18,6 +18,7 @@ This is the Serverless Framework plugin for AWS Step Functions.
      - [Pre-deployment validation](#pre-deployment-validation)
      - [Express Workflow](#express-workflow)
      - [CloudWatch Logs](#cloudwatch-logs)
+     - [X-Ray](#x-ray)
  - [Current Gotcha](#current-gotcha)
  - [Events](#events)
      - [API Gateway](#api-gateway)
@@ -407,6 +408,18 @@ stepFunctions:
           - Fn::GetAtt: [MyLogGroup, Arn]
 ```
 
+### X-Ray
+
+You can enable X-Ray for your state machine, specify `tracingConfig` as shown below.
+
+```yaml
+stepFunctions:
+  stateMachines:
+    hellostepfunc1:
+      tracingConfig:
+        enabled: true
+```
+
 ## Current Gotcha
 
 Please keep this gotcha in mind if you want to reference the `name` from the `resources` section. To generate Logical ID for CloudFormation, the plugin transforms the specified name in serverless.yml based on the following scheme.