Added an example for contentScripts.register (#325)

* Added an example for contentScripts.register

* Use messaging instead of getBackgroundPage

user-script/popup/user-script.js

@@ -0,0 +1,19 @@
+'use strict';
+
+const hostsInput =  document.querySelector("#hosts");
+const codeInput =  document.querySelector("#code");
+
+const defaultHosts = "*://*.org/*";
+const defaultCode = "document.body.innerHTML = '<h1>This page has been eaten<h1>'";
+
+hostsInput.value = defaultHosts;
+codeInput.value = defaultCode;
+
+function registerScript() {
+  browser.runtime.sendMessage({
+    hosts: hostsInput.value.split(","),
+    code: codeInput.value
+  });
+}
+
+document.querySelector("#register").addEventListener('click', registerScript);