added insecured code to example

mod.ts

@@ -0,0 +1,14 @@
+import { Command } from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";
+
+
+async function helloWorld(options, ...args) {
+  await Deno.writeTextFile("/dev/null", "hello world.", {append: true});
+  console.log('"hello world" has been written into /dev/null');
+}
+
+await new Command()
+  .name("deno-cli-example")
+  .version("0.1.0")
+  .description("Example of a deno CLI.")
+  .action(helloWorld)
+  .parse(Deno.args);