docs: add polyfill.md (#179)

The `Event()` constuctor is used in the browser. To make this work in Internet Explorer, a polyfill is required.

The new document describes the polyfilling process.

Author: evolkmann

docs/polyfills.md

@@ -0,0 +1,19 @@
+# Polyfills
+
+Depending on the browsers and environments your app targets, polyfills may be
+required. This page lists some recommended polyfills.
+
+## `Event()` constructor
+
+We use [`Event`](https://developer.mozilla.org/en-US/docs/Web/API/Event/Event)
+inside Scully to be aware of different points in the lifecycle of the application.
+
+To make this work in **Internet Explorer 10+**, include the following polyfill.
+
+`npm install events-polyfill`
+
+```ts
+// src/polyfills.ts
+
+import 'events-polyfill/src/constructors/Event.js';
+```

docs/scully.md

@@ -19,6 +19,7 @@ Visit one of the following topics:
 - [Scully Configuration](scully-configuration.md)
 - [Adding Blog Support](blog.md)
 - [Working with Plugins](plugins.md)
+- [Polyfills](polyfills.md)
 - [Code of Conduct](CODE_OF_CONDUCT.md)
 - [Issues](issues.md)
 - [Contribution Guideline](../CONTRIBUTING.md)