README.md: Add missing link

Author: AlfioEmanueleFresta

README.md

@@ -51,6 +51,7 @@ Contributions are very welcome!
 
 If you'd like to contribute but you don't know where to start, check out the _Issues_ tab of [each repository][#repositories].
 
+[xdg-portal]: https://flatpak.github.io/xdg-desktop-portal/portal-docs.html
 [linux-credentials]: https://github.com/linux-credentials
 [webauthn]: https://www.w3.org/TR/webauthn/
 [firefox-hello]: https://blog.mozilla.org/security/2019/03/19/passwordless-web-authentication-support-via-windows-hello/