docs(README): add note about timeoutFn

Author: addyosmani

README.md

@@ -6,7 +6,7 @@
 Quicklink attempts to make navigations to subsequent pages load faster. It:
 
 * **Detects links within the viewport** (using [Intersection Observer](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API))
-* **Waits until the browser is idle** (using [requestIdleCallback](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback))
+* **Waits until the browser is idle** (using [requestIdleCallback](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback)) or **waits until network activity is idle** (using [networkIdleCallback](https://github.com/pastelsky/network-idle-callback))
 * **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>](https://www.w3.org/TR/resource-hints/#prefetch) or XHR). Provides some control over the request priority (can switch to `fetch()` if supported).
 
@@ -53,6 +53,7 @@ The above options are best for multi-page sites. Single-page apps have a few opt
 * `el`: DOM element to observe for in-viewport links to prefetch
 * `urls`: Static array of URLs to prefetch (instead of observing `document` or a DOM element links in the viewport)
 * `timeout`: Integer for the `requestIdleCallback` timeout. A time in milliseconds by which the browser must execute prefetching. Defaults to 2 seconds.
+* `timeoutFn`: Function for specifying a timeout. Defaults to `requestIdleCallback`. Can also be swapped out for a custom function like [networkIdleCallback](https://github.com/pastelsky/network-idle-callback) (see demos)
 * `priority`: String specifying preferred priority for fetches. Defaults to `low`. `high` will attempt to use the `fetch()` API where supported (rather than rel=prefetch)
 
 TODO: