add warning about code injection

Author: wing328

README.md

@@ -8,6 +8,8 @@
 
 :notebook_with_decorative_cover: For more information, please refer to the [Wiki page](https://github.com/swagger-api/swagger-codegen/wiki) and [FAQ](https://github.com/swagger-api/swagger-codegen/wiki/FAQ) :notebook_with_decorative_cover:
 
+:warning: If the OpenAPI/Swagger spec is obtained from an untrusted source, please make sure you've reviewed the spec before using Swagger Codegen to generate the API client, server stub or documentation as [code injection](https://en.wikipedia.org/wiki/Code_injection) may occur :warning:
+
 ## Overview
 This is the swagger codegen project, which allows generation of client libraries automatically from a Swagger-compliant server.