docs(README): add how it works, usage and recipes.

Author: addyosmani

README.md

@@ -1,2 +1,69 @@
 # quicklink
-Faster web page loads by prefetching in-viewport links in the background
+> Faster subsequent page-loads by prefetching in-viewport links in the background
+
+## How it works
+
+Quicklink makes subsequent navigations to pages load faster. It:
+
+* **Detects links within the viewport** (using `IntersectionObsever`)
+* **Waits until the browser is idle** (using `requestIdleCallback`)
+* **Checks if the user isn't on a slow connection** (using `navigator.connection.effectiveType`) or has data-saver enabled (using `navigator.connection.saveData`)
+* **Prefetches URLs to the links** (using `<link rel=prefetch>` or XHR). Provides some control over the request priority (can switch to `fetch()` if supported).
+
+
+## Usage
+
+Quickstart:
+
+```html
+<!-- Include quicklink from dist -->
+<script src="dist/quicklink.js"></script>
+<!-- Initialize (you can do this to whenever you want) -->
+<script>
+quicklink();
+</script>
+```
+
+ES Module import:
+
+```js
+import quicklink from "dist/quicklink.mjs";
+quicklink();
+```
+
+## Recipes
+
+**Set the DOM element to obseve for in-viewport links**
+
+Defaults to `document` otherwise.
+
+```js
+let elem = document.getElementById('stuff');
+quicklink({
+  el: elem
+});
+```
+
+**Set a custom array of URLs to be prefetched**
+
+If you would prefer to provide a static list of URLs to be prefetched, instead of detecting those in-viewport, customizing URLs is supported.
+
+```js
+quicklink({
+   urls: ['2.html','4.html']
+});
+```
+
+**Set the request priority for prefetches**
+
+Defaults to low-priority (`rel=prefetch` or XHR). For high-priority,
+attempts to use fetch() or falls back to XHR.
+
+```js
+quicklink({ priority: 'high' });
+```
+
+## License
+
+Licensed under the Apache-2.0 license.
+