docs(reactive-forms): restore KW's paragraph that I deleted by accident

Author: wardbell

public/docs/ts/latest/guide/reactive-forms.jade

@@ -60,6 +60,17 @@ a#intro
   are always synchronous and under your control. 
   You won't encounter the timing issues that sometimes plague a template-driven form.
 
+  In keeping with the reactive paradigm, the component 
+  preserves the immutability of the _data model_,
+  treating it as a pure source of original values.
+  Rather than update the data model directly, 		
+  the component extracts user changes and forwards them to an external component or service,		
+  which does something with them (such as saving them) 		
+  and returns a new _data model_ to the component that reflects the updated model state.		
+    
+  Using reactive form directives does not require you to follow all reactive priniciples,		
+  but it does facilitate the reactive programming approach should you choose to use it.
+
   ### _Template-driven_ forms
 
   _Template-driven_ forms, introduced in the [Template guide](forms.html), take a completely different approach.