Add help to main command

Signed-off-by: Jacob Kiers <code@kiers.eu>

src/main.rs

@@ -5,6 +5,7 @@ mod upstreams;
 
 use crate::config::ConfigV1;
 use crate::servers::Server;
+use std::io::{stderr, stdout, Write};
 
 use log::{debug, error, info};
 use pico_args::Arguments;
@@ -16,17 +17,35 @@ fn main() {
     match args.subcommand().expect("Unexpected error").as_deref() {
         Some("serve") => serve(),
         Some("update") => update::update(),
+        Some("help") => {
+            let _ = print_usage(&mut stdout().lock());
+        }
         Some(cmd) => {
             eprintln!("Invalid command: {cmd}");
             std::process::exit(1);
         }
         None => {
             eprintln!("Calling l4p without argument is deprecated now. Please use: l4p serve");
+            let _ = print_usage(&mut stderr().lock());
             serve();
         }
     }
 }
 
+fn print_usage(out: &mut dyn Write) -> std::io::Result<()> {
+    writeln!(
+        out,
+        "{} v{}",
+        env!("CARGO_PKG_NAME"),
+        env!("CARGO_PKG_VERSION")
+    )?;
+    writeln!(out, "Usage:")?;
+    writeln!(out, "\tupdate\tUpdate l4p to the latest version")?;
+    writeln!(out, "\tserve\tServe the proxy")?;
+    writeln!(out, "\thelp\tPrint this message")?;
+    Ok(())
+}
+
 fn serve() {
     let config_path = match find_config() {
         Ok(p) => p,