Update the directives section

Author: mgechev

README.md

@@ -146,7 +146,8 @@ With `div#child` is associated `ChildCtrl` but since the scope injected inside `
 
 ### Directives
 
-In AngularJS the directives are the place where all DOM manipulations should be encapsulated. Each directive has a name and logic associated with it. In the simplest case the directive contains only name and definition of *postLink* function, which encapsulates all the logic required for the directive. In more complex cases the directive could contain a lot of properties such as:
+In AngularJS the directives are the place where all DOM manipulations should be placed. As a rule of thumb, when you have DOM manipulations in your controller you should create a new direcrive or consider refactoring of already existing one, which could handle the required DOM manipulations.
+Each directive has a name and logic associated with it. In the simplest case the directive contains only name and definition of *postLink* function, which encapsulates all the logic required for the directive. In more complex cases the directive could contain a lot of properties such as:
 
 - template
 - compile function