First pass at documentation

README.md

@@ -260,6 +260,46 @@ response.headers['X-Request-URL'] = request.url
 This server workaround is necessary if you need reliable `response.url` in
 Firefox < 32, Chrome < 37, Safari, or IE.
 
+#### Aborting requests
+
+This polyfill supports
+[the abortable fetch API](https://developers.google.com/web/updates/2017/09/abortable-fetch). However,
+aborting a fetch requires use of two additional DOM APIs:
+[AbortController](https://developer.mozilla.org/en-US/docs/Web/API/AbortController)
+and [AbortSignal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal). Typically,
+browsers that do not support fetch will also not support AbortController or AbortSignal.
+
+Consequently, you will need to find a polyfill for these APIs to
+abort fetches.
+
+Once you have an AbortController and AbortSignal polyfill in place, you can abort a fetch like
+so:
+
+```js
+const controller = new AbortController()
+
+fetch('/avatars', {
+  method: 'POST',
+  body: data,
+  signal: controller.signal
+})
+  .then(
+    function(response) {
+      console.log('request succeeded', response)
+    },
+    function(ex) {
+      if (ex.name === 'AbortError') {
+        console.log('request aborted')
+      } else {
+        console.log('request failed, but it was not aborted')
+      }
+    }
+  )
+
+// some time later...
+controller.abort();
+```
+
 ## Browser Support
 
 - Chrome