Add Fig as an installation method to the README (#459)

README.md

@@ -229,6 +229,12 @@ atuin init fish | source
 
 to your `is-interactive` block in your `~/.config/fish/config.fish` file
   
+### Fig
+
+Install `atuin` shell plugin in zsh, bash, or fish with [Fig](https://fig.io) in one click. 
+
+<a href="https://fig.io/plugins/other/atuin" target="_blank"><img src="https://fig.io/badges/install-with-fig.svg" /></a>
+
 ## ...what's with the name?
 
 Atuin is named after "The Great A'Tuin", a giant turtle from Terry Pratchett's