Update README.md

Author: philsturgeon

README.md

@@ -2,6 +2,9 @@ Swagger Express Middleware
 ============================
 ### Swagger 2.0 middleware and mocks for Express.js
 
+> :warning: **No longer maintained**
+> This package has been abandoned for some time, and there are plenty of modern alternatives around which support OpenAPI v3.x listed on [OpenAPI.Tools](https://openapi.tools/) which you can use instead.
+
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 [![Build Status](https://github.com/APIDevTools/swagger-express-middleware/workflows/CI-CD/badge.svg?branch=master)](https://github.com/APIDevTools/swagger-express-middleware/actions)