Link to manifest-app-info in the README (#1030)

Fix #1027.

Author: xfq

README.md

@@ -4,5 +4,6 @@ the [Web Applications Working Group](https://www.w3.org/2019/webapps/).
 ## Useful links
 * [Explainer](https://github.com/w3c/manifest/blob/gh-pages/explainer.md)
 * [The Web App Manifest specification](https://www.w3.org/TR/appmanifest/)
+* [App Information supplement](https://github.com/w3c/manifest-app-info)
 * [Use cases and requirements](https://w3c-webmob.github.io/installable-webapps/)
 * [The Web Applications WG homepage](https://www.w3.org/2019/webapps/)