Add shallow clone flag to install section

Just adding `--depth=1` to the git clone command for folks copy-pasting the install directions. Figured it might save users a few bits of disk and network usage that they might not otherwise think about.

README.md

@@ -93,7 +93,7 @@ sudo apt install quickemu
 ## Other Linux
 
 ```bash
-git clone https://github.com/wimpysworld/quickemu
+git clone --depth=1 https://github.com/wimpysworld/quickemu
 cd quickemu
 ```