Document onerror callback.

README.md

@@ -147,6 +147,24 @@ getJSON("/post/1.json").then(function(post) {
 });
 ```
 
+## Error Handling
+
+There are times when dealing with promises that it seems like any errors
+are being 'swallowed', and not properly raised. This makes is extremely
+difficult to track down where a given issue is coming from. Thankfully,
+`RSVP` has a solution for this problem built in.
+
+You can provide an `onerror` function that will be called with the error
+details if any errors occur within your promise. This function can be anything
+but a common practice is to call `console.assert` to dump the error to the
+console.
+
+```javascript
+RSVP.configure('onerror', function(error) {
+  console.assert(false, error);
+});
+```
+
 ## Arrays of promises
 
 Sometimes you might want to work with many promises at once. If you