add completion install to README

README.MD

@@ -26,6 +26,7 @@ Options:
   --character, -c char        Character to use to draw the flag
   --gradient, -g              Make the flag a smooth gradient
   --help, -h                  Display this help text
+  --install-completion        Install tabtab shell completion
   --live, -l                  Hold the terminal and redraw the flag upon resize, closing when any key is pressed
   --random, -r                Displays a random flag! This ignores any passed flags.
   --vertical, -v              Display the flag, but vertically