readme: mention the new gnome-nightly image

Author: bilelmoussaoui

README.md

@@ -102,5 +102,6 @@ You can specify the specific runtime you need to use through the image tags:
 | Freedesktop SDK | 20.08   | `freedesktop-20.08` | `image: bilelmoussaoui/flatpak-github-actions:freedesktop-20.08` |
 | GNOME           | 3.38    | `gnome-3.38`        | `image: bilelmoussaoui/flatpak-github-actions:gnome-3.38`        |
 | GNOME           | 40    | `gnome-40`        | `image: bilelmoussaoui/flatpak-github-actions:gnome-40`        |
+| GNOME           | master    | `gnome-nightly`        | `image: bilelmoussaoui/flatpak-github-actions:gnome-nightly`        |
 | KDE             | 5.15    | `kde-5.15`          | `image: bilelmoussaoui/flatpak-github-actions:kde-5.15`          |
 | elementary BaseApp             | juno    | `juno`          | `image: bilelmoussaoui/flatpak-github-actions:elementary-juno`          |