nl: move help strings to a markdown file (#4477)

src/uu/nl/nl.md

@@ -0,0 +1,7 @@
+#nl
+
+```
+nl [OPTION]... [FILE]...
+```
+
+Number lines of files

src/uu/nl/src/nl.rs

@@ -14,12 +14,12 @@ use std::io::{stdin, BufRead, BufReader, Read};
 use std::iter::repeat;
 use std::path::Path;
 use uucore::error::{FromIo, UResult, USimpleError};
-use uucore::format_usage;
+use uucore::{format_usage, help_about, help_usage};
 
 mod helper;
 
-static ABOUT: &str = "Number lines of files";
-static USAGE: &str = "{} [OPTION]... [FILE]...";
+static ABOUT: &str = help_about!("nl.md");
+static USAGE: &str = help_usage!("nl.md");
 
 // Settings store options used by nl to produce its output.
 pub struct Settings {