Added an example for contentScripts.register

user-script/background.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var registered = null;
+
+async function register(hosts, code) {
+
+  if (registered) {
+    registered.unregister();
+  }
+
+  registered = await browser.contentScripts.register({
+    matches: hosts,
+    js: [{code}],
+    runAt: "document_idle"
+  });
+
+}