Add docs for retry interval function

Author: aianus

README.md

@@ -1571,6 +1571,7 @@ __Arguments__
 * `opts` - Can be either an object with `times` and `interval` or a number.
   * `times` - The number of attempts to make before giving up.  The default is `5`.
   * `interval` - The time to wait between retries, in milliseconds.  The default is `0`.
+    The interval may also be specified as a function of the retry count.
   * If `opts` is a number, the number specifies the number of times to retry, with the default interval of `0`.
 * `task(callback, results)` - A function which receives two arguments: (1) a `callback(err, result)`
   which must be called when finished, passing `err` (which can be `null`) and the `result` of
@@ -1595,6 +1596,21 @@ async.retry({times: 3, interval: 200}, apiMethod, function(err, result) {
 });
 ```
 
+The interval may also be specified as a function of the retry count:
+
+```js
+// try calling apiMethod 10 times with exponential backoff
+// (i.e. intervals of 100, 200, 400, 800, 1600, ... milliseconds)
+async.retry({
+  times: 10,
+  interval: function(retryCount) {
+    return 50 * Math.pow(2, retryCount);
+  }
+}, apiMethod, function(err, result) {
+    // do something with the result
+});
+```
+
 ```js
 // try calling apiMethod the default 5 times no delay between each retry
 async.retry(apiMethod, function(err, result) {

lib/retry.js

@@ -19,7 +19,8 @@ import constant from 'lodash/constant';
  * * `times` - The number of attempts to make before giving up.  The default
  *   is `5`.
  * * `interval` - The time to wait between retries, in milliseconds.  The
- *   default is `0`.
+ *   default is `0`. The interval may also be specified as a function of the
+ *   retry count.
  * * If `opts` is a number, the number specifies the number of times to retry,
  *   with the default interval of `0`.
  * @param {Function} task - A function which receives two arguments: (1) a
@@ -47,7 +48,18 @@ import constant from 'lodash/constant';
  *     // do something with the result
  * });
  *
- *  // try calling apiMethod the default 5 times no delay between each retry
+ * // try calling apiMethod 10 times with exponential backoff
+ * // (i.e. intervals of 100, 200, 400, 800, 1600, ... milliseconds)
+ * async.retry({
+ *   times: 10,
+ *   interval: function(retryCount) {
+ *     return 50 * Math.pow(2, retryCount);
+ *   }
+ * }, apiMethod, function(err, result) {
+ *     // do something with the result
+ * });
+ *
+ * // try calling apiMethod the default 5 times no delay between each retry
  * async.retry(apiMethod, function(err, result) {
  *     // do something with the result
  * });