Add Flathub badge (#5091)

Author: uglide

README.md

@@ -17,6 +17,7 @@ Open source cross-platform Desktop Manager for Redis ® based on Qt 5
 <a href="https://snapcraft.io/redis-desktop-manager">
 <img height="50" alt="Get it from the Snap Store" src="https://snapcraft.io/static/images/badges/en/snap-store-black.svg" />
 </a>
+<a href='https://flathub.org/apps/details/dev.rdm.RDM'><img height="50" alt='Download on Flathub' src='https://flathub.org/assets/badges/flathub-badge-en.png'/></a>
 
 ![RDM screenshot](http://rdm.dev/static/img/features/all.png?v2021)