Add a link to eBook on top (#7588)

The eBook is hard to find as it's in the presentation section (bottom)

Author: chrishiggins34

README.md

@@ -21,6 +21,8 @@
 
 :rocket: ProductHunt: https://producthunt.com/posts/swagger-codegen :rocket:
 
+:notebook_with_decorative_cover: The eBook [A Beginner's Guide to Code Generation for REST APIs](https://gumroad.com/l/swagger_codegen_beginner) is a good starting point for begineers.
+
 ## Overview
 This is the swagger codegen project, which allows generation of API client libraries (SDK generation), server stubs and documentation automatically given an [OpenAPI Spec](https://github.com/OAI/OpenAPI-Specification). Currently, the following languages/frameworks are supported: