add note about named watch functions to FAQ

FAQ.md

@@ -2,3 +2,14 @@
 
 ### How do I measure a directive's performance?
 If your directive uses `$watch`, you should be able to see the watch expression wherever your directive is used.
+
+### My $watch functions show up as just "function ()" in the performance tab
+Use named functions for $watch:
+
+```javascript
+scope.$watch(function checkIfSomethingChanged() { 
+  // ...
+}, function whenThatChanges(newValue, oldValue) {
+  // ...
+});
+```