Update docs

README.md

@@ -180,6 +180,7 @@ parameters have their XHR default values:
 | [`responseType`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/responseType)         | `''`        | Specifies type of [response](#XHR-response) data.                                                                                                                                                                                                                                                  |
 | [`timeout`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/timeout)                   | `0`         | Number of milliseconds or `0` for infinite.                                                                                                                                                                                                                                                        |
 | [`withCredentials`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/withCredentials)   | `false`     | Whether cross-site `Access-Control` should use credentials.                                                                                                                                                                                                                                        |
+| [`onAbort`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/abort)                     | `undefined` | A function to be called if the request is aborted as a consequence of the observable being unsubscribed.                                                                                                                                                                                           |
 
 In addition to a plain object, the argument to `XHR.perform` is allowed to be an
 observable property or contain observable properties, in which case the property