added file header

src/main.rs

@@ -135,8 +135,10 @@ fn main() {
     let conf = toml::from_str::<Table>(&conf_file)
         .unwrap_or_else(|e| panic!("fatal: could not parse config file: {e}"));
 
-    let mut ninjafile = String::from(
-        r#"builddir = build
+    let mut ninjafile = String::from(&format!(
+        r#"# Generated by ngen. Do not modify by hand.
+
+builddir = build
 
 rule mkdir
   command = mkdir -p $out
@@ -150,7 +152,7 @@ rule regen
 build build.ninja: regen ngen.toml
   pool = console
 "#,
-    );
+    ));
 
     for k in conf.keys() {
         match conf.get(k) {