Document how to generate prefixed completions (#8817)

* Document how to generate prefixed completions

Author: oech3

README.md

@@ -256,6 +256,11 @@ So, to install completions for `ls` on `bash` to
 cargo run completion ls bash > /usr/local/share/bash-completion/completions/ls
 ```
 
+Completion for prefixed `cp` with `uu-` on `zsh` is generated by
+```shell
+env PROG_PREFIX=uu- cargo run completion cp zsh
+```
+
 ### Manually install manpages
 
 To generate manpages, the syntax is: