Adding cancelrequest doc (microsoftgraph#409)

Author: nikithauc

docs/CancellingAHTTPRequest.md

@@ -0,0 +1,41 @@
+# Cancel a HTTP request
+
+> The `abort()` method of the AbortController interface aborts a DOM request (e.g. a Fetch request) 
+> 
+> -- [AbortController interface](https://developer.mozilla.org/en-US/docs/Web/API/AbortController)
+
+References - 
+* [AbortController interface](https://developer.mozilla.org/en-US/docs/Web/API/AbortController)
+* [abortcontroller npm](https://www.npmjs.com/package/abort-controller)
+* [abortcontroller-polyfill](https://www.npmjs.com/package/abortcontroller-polyfill)
+* [Example of the AbortController implementation](https://github.com/node-fetch/node-fetch#request-cancellation-with-abortsignal)
+
+#### Following is how canceling a fetch call works:
+
+* Create an AbortController instance.
+* That instance has a signal property.
+* Pass the signal as a fetch option for signal.
+* Call the AbortController's abort property to cancel all fetches that use that signal.
+
+#### Setting the AbortController.signal as a fetch option while creating the MSGraph SDK Client instance:
+
+```typescript
+import { Client,FetchOptions } from "@microsoft/microsoft-graph-client";
+import { AbortController } from "abort-controller"; // <- import when using the abort-controller npm package.
+
+const controller = new AbortController();
+
+const timeout = setTimeout(() => {
+	controller.abort();
+}, 150);
+
+const fetchOptions: FetchOptions = {
+	signal: controller.signal;
+}
+
+const client = Client.initWithMiddleware({
+  fetchOptions, // Pass the FetchOptions value where the AbortController.signal is set
+  authProvider,
+  ...
+});
+```