Add parse-server-mailgun-adapter-template to README.md (#2413)

Customize the e-mail sent to your users when they reset their password or when we verify their email address.

Author: bcomeau

README.md

@@ -267,6 +267,7 @@ You can also use other email adapters contributed by the community such as:
 - [parse-server-sendgrid-adapter](https://www.npmjs.com/package/parse-server-sendgrid-adapter)
 - [parse-server-mandrill-adapter](https://www.npmjs.com/package/parse-server-mandrill-adapter)
 - [parse-server-simple-ses-adapter](https://www.npmjs.com/package/parse-server-simple-ses-adapter)
+- [parse-server-mailgun-adapter-template](https://www.npmjs.com/package/parse-server-mailgun-adapter-template)
 
 ### Using environment variables to configure Parse Server