mention shell completions usage in README.md

[scarf005]

lsd/build.rs

Lines 24 to 36 in 2fe3fcd

	let outdir = std::env::var_os("SHELL_COMPLETIONS_DIR")
	.or_else(|| std::env::var_os("OUT_DIR"))
	.unwrap_or_else(|| exit(0));
	fs::create_dir_all(&outdir).unwrap();
	let mut app = Cli::command();
	let bin_name = "lsd";
	generate_to(Bash, &mut app, bin_name, &outdir).expect("Failed to generate Bash completions");
	generate_to(Fish, &mut app, bin_name, &outdir).expect("Failed to generate Fish completions");
	generate_to(Zsh, &mut app, bin_name, &outdir).expect("Failed to generate Zsh completions");
	generate_to(PowerShell, &mut app, bin_name, &outdir)
	.expect("Failed to generate PowerShell completions");

despite lsd providing shell completions via clap_complete::generate_to, there is no documentation that either SHELL_COMPLETIONS_DIR or OUT_DIR environment variable needs to be set to generate one.

alternatively, LSD could provide a command-line option to generate completions at runtime.

[zwpaper]

/kind doc
/good-first-issue
/help

[zwpaper]

/good-first-issue
/help

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