README.md: add warning when deleting stacks

Author: thomas-roos

README.md

@@ -39,6 +39,8 @@ pipelines will work correctly. Once it is complete, the EmbeddedLinuxPipeline in
 
 ### Removing Pipelines
 The `cdk destroy` command can be used to remove individual pipelines and their related resources. This can also be done in the CloudFormation Console Page.
+[!WARNING]
+Do not delete stacks while a CodePipeline is running this can lead to unexpected failures!
 
 To remove all the resources associated with this application:
 ```bash