Adding new recipe to retrywhen

Author: mattpodwysocki

doc/api/core/operators/retrywhen.md

@@ -123,6 +123,29 @@ var subscription = source.subscribe(
 // => Completed
 ```
 
+An incrememntal back-off strategy for handling errors:
+```js
+Rx.Observable.create(function (o) {
+    console.log("subscribing");
+    o.onError(new Error("always fails"));
+  }).retryWhen(function (attempts) {
+      return Rx.Observable.range(1, 3).zip(attempts, function (i) { return i; }).flatMap(function (i) {
+        console.log("delay retry by " + i + " second(s)");
+        return Rx.Observable.timer(i * 1000);
+      });
+  }).subscribe();
+  
+/*
+subscribing
+delay retry by 1 second(s)
+subscribing
+delay retry by 2 second(s)
+subscribing
+delay retry by 3 second(s)
+subscribing
+*/
+```
+
 ### Location
 
 File: