Add a doc describing howto enable and use the API.

Author: wanderview

howto.md

@@ -0,0 +1,19 @@
+# How to use the URLPattern API
+
+The URLPattern API is currently available in Chrome 93 and later behind a flag.
+
+1. Download Chrome ([Android](https://play.google.com/store/apps/details?id=com.android.chrome), [Desktop](https://www.google.com/chrome/)).
+2. Navigate to `chrome://flags` and enable `Experimental Web Platform features`.
+3. Visit a page that uses `URLPattern`, such as the [demo on glitch](https://urlpattern-sw-routing.glitch.me/).
+
+Here is an example of how to use the API:
+
+```javascript
+const pattern = new URLPattern({ pathname: '/:product/index.html' });
+const result = pattern.exec(someURL);
+if (result) {
+  handleProduct(result.pathname.groups.product);
+}
+```
+
+More code examples can be found in the [URLPattern documentation](docs/urlpattern.md).