docs(CHANGELOG.md): add breaking change from 5f3f25a1

5f3f25a1 included a breaking change which was not documented, which is that the return value of directive
constructors is ignored. The reason they are ignored is to ensure that the correct object is bound to when
binding properties to the controller. It may be possible to come up with a better solution which informs
the developer that what they are doing is wrong, rather than just breaking instead.

Closes #8876
Closes #8882

CHANGELOG.md

@@ -317,7 +317,50 @@ Closes #8803
 Closes #6910
 Closes #5402
 
+- **$compile:** due to [5f3f25a1](https://github.com/angular/angular.js/commit/5f3f25a1a6f9d4f2a66e2700df3b9c5606f1c255),
 
+The returned value from directive controller constructors are now ignored, and only the constructed
+instance itself will be attached to the node's expando. This change is necessary in order to ensure
+that it's possible to bind properties to the controller's instance before the actual constructor is
+invoked, as a convenience to developers.
+
+In the past, the following would have worked:
+
+```js
+angular.module("myApp", []).
+    directive("myDirective", function() {
+        return {
+            controller: function($scope) {
+                return {
+                    doAThing: function() { $scope.thingDone = true; },
+                    undoAThing: function() { $scope.thingDone = false; }
+                };
+            },
+            link: function(scope, element, attrs, ctrl) {
+                ctrl.doAThing();
+            }
+        };
+    });
+```
+
+However now, the reference to `doAThing()` will be undefined, because the return value of the controller's constructor is ignored. In order to work around this, one can opt for several strategies, including the use of `_.extend()` or `merge()` like routines, like so:
+
+```js
+angular.module("myApp", []).
+    directive("myDirective", function() {
+        return {
+            controller: function($scope) {
+                _.extend(this, {
+                    doAThing: function() { $scope.thingDone = true; },
+                    undoAThing: function() { $scope.thingDone = false; }
+                });
+            },
+            link: function(scope, element, attrs, ctrl) {
+                ctrl.doAThing();
+            }
+        };
+    });
+```
 
 <a name="1.2.23"></a>
 # 1.2.23 superficial-malady (2014-08-22)