docs(README.md): add purpose section

Add a new purpose section to enable newcomers (technical and non-technical)
better understand the purpose of AngularJS

Close #13963

Author: aashishnagpal

README.md

@@ -18,7 +18,7 @@ piece of cake. Best of all?? It makes development fun!
 * Developer Guide: http://docs.angularjs.org/guide
 * Contribution guidelines: [CONTRIBUTING.md](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md)
 * Dashboard: http://dashboard.angularjs.org
-
+ 
 Building AngularJS
 ---------
 [Once you have your environment set up](http://docs.angularjs.org/misc/contribute) just run:
@@ -43,3 +43,26 @@ To learn more about the grunt tasks, run `grunt --help` and also read our
 
 [![Analytics](https://ga-beacon.appspot.com/UA-8594346-11/angular.js/README.md?pixel)](https://github.com/igrigorik/ga-beacon)
 
+What to Use AngularJS for and When to Use it
+---------
+AngularJS is the next generation framework where each component is designed to work with every other component in an interconnected way like a well-oiled machine. AngularJS is JavaScript MVC made easy and done right. (Well it is not really MVC, read on, to understand what this means.)
+
+#### MVC, no, MV* done the right way!
+MVC, short for Model-View-Controller, is a design pattern, i.e. how the code should be organized and how the different parts of an application separated for proper readability and debugging. Model is the data and the database. View is the user interface and what the user sees. Controller is the main link between Model and View. These are the three pillars of major programming frameworks present on the market today. On the other hand AngularJS works on MV*, short for Model-View-_Whatever_. The _Whatever_ is AngularJS's way of telling that you may create any kind of linking between the Model and the View here. 
+
+Unlike other frameworks in any programming language, where MVC, the three separate components, each one has to be written and then connected by the programmer, AngularJS helps the programmer by asking him/her to just create these and everything else will be taken care of by AngularJS.
+
+#### Interconnection with HTML at the root level
+AngularJS uses HTML to define the user's interface. AngularJS also enables the programmer to write new HTML tags (AngularJS Directives) and increase the readability and understandability of the HTML code. Directives are AngularJS’s way of bringing additional functionality to HTML. Directives achieve this by enabling us to invent our own HTML elements. This also helps in making the code DRY (Don't Repeat Yourself), which means once created, a new directive can be used anywhere within the application.
+
+#### Data Handling made simple
+Data and Data Models in AngularJS are plain JavaScript objects and one can add and change properties directly on it and loop over objects and arrays at will.
+
+#### Two-way Data Binding
+One of AngularJS's strongest features. Two-way Data Binding means that if something changes in the Model, the change gets reflected in the View instantaneously, and the same happens the other way around. This is also referred to as Reactive Programming, i.e. suppose `a = b + c` is being programmed and after this, if the value of `b` and/or `c` is changed then the value of `a` will be automatically updated to reflect the change. AngularJS uses its "scopes" as a glue between the Model and View and makes these updates in one available for the other.
+
+#### Less Written Code and Easily Maintable Code
+Everything in AngularJS is created to enable the programmer ends up writing less code that is easily maintainable and readable by any other new person on the team. Believe it or not, one can write a complete working two-way data binded application in less than 10 lines of code. Try and see for yourself!
+
+#### Testing Ready
+AngularJS has Dependency Injection, i.e. it takes care of providing all the necessary dependencies to its controllers whenever required. This helps in making the AngularJS code ready for unit testing by making use of mock dependencies created and injected. This makes AngularJS more modular and easily testable thus in turn helping a team create more robust applications.