Add flathub bagde to README.md
depends on #153 (closed)
Add flathub link button on top (under title)
<a href="https://flathub.org/apps/details/org.gnome.gitg"><img height="51" alt="Download on Flathub" src="https://flathub.org/assets/badges/flathub-badge-en.svg"/></a>
See https://twitter.com/JorgeEscriu/status/1055031442680496128 from @jgarciao
Edited by Ghost User