update README

README.md

@@ -3,13 +3,17 @@
 Like calling `map()` and resolving with `Promise.all()`,
 but handling a bounded number of items at any given time.
 
-## Install
+Want to read about it?
+
+- [Batching async requests in < 50 lines of VanillaJS](https://coolaj86.com/articles/batching-async-requests-50-lines-of-vanilla-js/)
+
+# Install
 
 ```bash
 npm install --save batchasync
 ```
 
-## Usage
+# Usage
 
 ```js
 // Browsers
@@ -35,10 +39,14 @@ batchAsync(batchSize, things, doStuff).then(function(results) {
 });
 ```
 
-### Returning 'undefined'
+## A note on 'undefined'
 
-Returning `undefined` will cause an exception to be thrown
-(because it looks like a skipped promise). Return `null` instead.
+Returning `undefined` will cause an exception to be thrown,
+and the Promise to be rejected.
+
+This is on purpose - because `undefined` is indistinguishable from a skipped Promise.
+
+Return `null` instead.
 
 **Example (bad)**: