added deprecation note

Author: mtrezza

README.md

@@ -1,4 +1,10 @@
-# parse-server-simple-mailgun-adapter
+# DEPRECATED
+
+This mail adapter has been deprecated. Instead, use one of the [community provided mail adapters](https://github.com/parse-community/parse-server#email-verification-and-password-reset).
+
+---
+# Parse Server Simple Mailgun Adapter
+
 <a href="https://www.npmjs.com/package/@parse/simple-mailgun-adapter"><img alt="npm version" src="https://img.shields.io/npm/v/@parse/simple-mailgun-adapter.svg?style=flat"></a>
 
 Used to send Parse Server password reset and email verification emails though Mailgun