Added cdk destroy demo

README.md

@@ -9,6 +9,7 @@ The `cdk.json` file tells the CDK Toolkit how to execute your app.
 * `npm run build`   compile typescript to js
 * `npm run watch`   watch for changes and compile
 * `npx cdk deploy`  deploy this stack to your default AWS account/region
+* `npx cdk destroy` delete this stack with all the resources it includes
 * `npx cdk diff`    compare deployed stack with current state
 * `npx cdk synth`   emits the synthesized CloudFormation template