Updated README

Signed-off-by: Mavridis Philippe <mavridisf@gmail.com>

README.md

@@ -4,8 +4,6 @@
 
 This is an implementation of the [FreeDesktop Portals API](https://flatpak.github.io/xdg-desktop-portal).
 
-THIS IS AN EARLY PROTOTYPE. USE WITH CARE.
-
 This implementation exposes TDE APIs to applications which use the Portals API. As a result, these applications
 can transparently use native APIs and dialogs.
 
@@ -14,6 +12,9 @@ Read more about the portals API on its official [website](https://flatpak.github
 ## How do I use this?
 Before using this, make sure you have the `xdg-desktop-portal` package installed and running on your system.
 
+Some applications respect the `GTK_USE_PORTAL=1` and `GDK_DEBUG=portals` environment variables (most GTK applications and
+some Chromium-based browsers, e.g. Vivaldi).
+
 To make Firefox use the Portals API, go to `about:config`, search for options that start with "widget.use-xdg-desktop-portal" and,
 if they are set to 2, set them to 1.