Add example using MV3 userScripts API (#576)

userScripts-mv3/userscript_examples/unprivileged.user.js

@@ -0,0 +1,20 @@
+// ==UserScript==
+// @name          Demo of unprivileged user script
+// @description   Show dialog on MDN and every URL starting with "https://example".
+// @match         https://developer.mozilla.org/*
+// @include       https://example*
+// @exclude-match https://example.com/display_userscript_result*
+// @grant         none
+// @version       1.2.3
+// ==/UserScript==
+
+// To test:
+// 1. Visit https://example.com/ or https://developer.mozilla.org/
+// 2. Confirm that a dialog shows up.
+
+alert(`This is a demo of a user script, running at ${document.URL}.`);
+
+// This user script should not get access to privileged APIs.
+if (typeof GM_info !== "undefined") {
+  alert("Unexpectedly, GM_info is defined...?");
+}