readme: Comment out flathub link

The app is not available yet.

Fixes: #16

README.md

-<a href="https://flathub.org/apps/details/org.gnome.design.Lettercase">
-<img src="https://flathub.org/assets/badges/flathub-badge-i-en.png" width="190px" />
-</a>
+<!-- <a href="https://flathub.org/apps/details/org.gnome.design.Lettercase"> -->
+<!-- <img src="https://flathub.org/assets/badges/flathub-badge-i-en.png" width="190px" /> -->
+<!-- </a> -->
 
 # Lettercase
 
@@ -16,6 +16,7 @@ Your new favorite font manager
 </div>
 
 ## Hack on Lettercase
+
 To build the development version of  Lettercase and hack on the code
 see the [general guide](https://wiki.gnome.org/Newcomers/BuildProject)
 for building GNOME apps with Flatpak and GNOME Builder.