adding docs about "fail" handler; removing redundant .call in fail()

README.md

@@ -116,7 +116,7 @@ Errors also propagate:
 ```javascript
 getJSON("/posts.json").then(function(posts) {
 
-}).fail(function(error) {
+}).then(null, function(error) {
   // since no rejection handler was passed to the
   // first `.then`, the error propagates.
 });
@@ -131,11 +131,22 @@ getJSON("/post/1.json").then(function(post) {
   return getJSON(post.commentURL);
 }).then(function(comments) {
   // proceed with access to posts and comments
-}).fail(function(error) {
+}).then(null, function(error) {
   // handle errors in either of the two requests
 });
 ```
 
+You can also use `fail` for error handling, which is a shortcut for
+`then(null, rejection)`, like so:
+
+```javascript
+getJSON("/post/1.json").then(function(post) {
+  return getJSON(post.commentURL);
+}).fail(function(error) {
+  // handle errors
+});
+```
+
 ## Arrays of promises
 
 Sometimes you might want to work with many promises at once. If you

lib/rsvp/promise.js

@@ -123,7 +123,7 @@ Promise.prototype = {
   },
 
   fail: function(fail) {
-    return this.then.call(this, null, fail);
+    return this.then(null, fail);
   }
 };