Add openSUSE (TW) to 'Installation' section

Since gitui is now available in the official software repository for openSUSE Tumbleweed it would make sense to add it to the install instructions.

Author: StefDev

README.md

@@ -118,6 +118,12 @@ sudo emerge --sync dm9pZCAq
 sudo emerge dev-vcs/gitui::dm9pZCAq
 ```
 
+### [openSUSE](https://software.opensuse.org/package/gitui) (Tumbleweed)
+
+```sh
+sudo zypper install gitui
+```
+
 ### Homebrew (macOS)
 
 ```sh