Add Parse Server Generic Email Adapter to README (#4101)

Added a generic email plugin to send Parse Server password reset and verification emails with generic accounts like Gmail. This module uses the nodemailer library.

Author: premithk

README.md

@@ -337,6 +337,7 @@ You can also use other email adapters contributed by the community such as:
 - [parse-server-sendinblue-adapter](https://www.npmjs.com/package/parse-server-sendinblue-adapter)
 - [parse-server-mailjet-adapter](https://www.npmjs.com/package/parse-server-mailjet-adapter)
 - [simple-parse-smtp-adapter](https://www.npmjs.com/package/simple-parse-smtp-adapter)
+- [parse-server-generic-email-adapter](https://www.npmjs.com/package/parse-server-generic-email-adapter)
 
 ### Using environment variables to configure Parse Server