Add self advertisement header

src/template.rs

@@ -120,6 +120,7 @@ impl Template {
             .chain_err(|| "Error while opening gitignore file to write template")?;
         let mut writer = BufWriter::new(file);
 
+        writer.write_all(b"# template downloaded with gitig (https://git.schneider-hosting.de/schneider/gitig) from https://github.com/github/gitignore\n")?;
         writer.write_all(self.content.as_ref().expect("checked before to be some").as_bytes())?;
         trace!("Wrote all content");
         Ok(())