Improve documentation of the expectations of a Batch Function.

Suggested in #66

Author: leebyron

README.md

@@ -54,6 +54,9 @@ unique cache. You might create each loader once for your whole application, or
 create new instances per request when used within a web-server like [express][]
 if different users can see different things. It's up to you.
 
+
+### Batching
+
 Batching is not an advanced feature, it's DataLoader's primary feature.
 Create loaders by providing a batch loading function.
 
@@ -64,7 +67,7 @@ var userLoader = new DataLoader(keys => myBatchGetUsers(keys));
 ```
 
 A batch loading function accepts an Array of keys, and returns a Promise which
-resolves to an Array of values.
+resolves to an Array of values[<sup>*</sup>](#batch-function).
 
 Then load individual values from the loader. DataLoader will coalesce all
 individual loads which occur within a single frame of execution (a single tick
@@ -92,6 +95,41 @@ presented to your batch loading function. This allows your application to safely
 distribute data fetching requirements throughout your application and maintain
 minimal outgoing data requests.
 
+#### Batch Function
+
+A batch loading function accepts an Array of keys, and returns a Promise which
+resolves to an Array of values. There are a few constraints that must be upheld:
+
+ * The Array of values must be the same length as the Array of keys.
+ * Each index in the Array of values must correspond to the same index in the Array of keys.
+
+For example, if your batch function was provided the Array of keys: `[ 2, 9, 6, 1 ]`,
+and loading from a back-end service returned the values:
+
+```js
+{ id: 9, name: 'Chicago' }
+{ id: 1, name: 'New York' }
+{ id: 2, name: 'San Francisco' }
+```
+
+Our back-end service returned results in a different order than we requested, likely
+because it was more efficient for it to do so. Also, it omitted a result for key `6`,
+which we can interpret as no value existing for that key.
+
+To uphold the constraints of the batch function, it must return an Array of values
+the same length as the Array of keys, and re-order them to ensure each index aligns
+with the original keys `[ 2, 9, 6, 1 ]`:
+
+```js
+[
+  { id: 2, name: 'San Francisco' },
+  { id: 9, name: 'Chicago' },
+  null,
+  { id: 1, name: 'New York' }
+]
+```
+
+
 ### Caching
 
 After being loaded once, the resulting value is cached, eliminating