Add Sandboxes for almost all examples (#2486)

Author: CompuIves

README.md

@@ -177,16 +177,18 @@ For Offline docs, please see: [devdocs](http://devdocs.io/redux/)
 
 ### Examples
 
+Almost all examples have a corresponding CodeSandbox sandbox. This is an interactive version of the code that you can play with online.
+
 * [Counter Vanilla](http://redux.js.org/docs/introduction/Examples.html#counter-vanilla) ([source](https://github.com/reactjs/redux/tree/master/examples/counter-vanilla))
-* [Counter](http://redux.js.org/docs/introduction/Examples.html#counter) ([source](https://github.com/reactjs/redux/tree/master/examples/counter))
-* [Todos](http://redux.js.org/docs/introduction/Examples.html#todos) ([source](https://github.com/reactjs/redux/tree/master/examples/todos))
-* [Todos with Undo](http://redux.js.org/docs/introduction/Examples.html#todos-with-undo) ([source](https://github.com/reactjs/redux/tree/master/examples/todos-with-undo))
-* [TodoMVC](http://redux.js.org/docs/introduction/Examples.html#todomvc) ([source](https://github.com/reactjs/redux/tree/master/examples/todomvc))
-* [Shopping Cart](http://redux.js.org/docs/introduction/Examples.html#shopping-cart) ([source](https://github.com/reactjs/redux/tree/master/examples/shopping-cart))
-* [Tree View](http://redux.js.org/docs/introduction/Examples.html#tree-view) ([source](https://github.com/reactjs/redux/tree/master/examples/tree-view))
-* [Async](http://redux.js.org/docs/introduction/Examples.html#async) ([source](https://github.com/reactjs/redux/tree/master/examples/async))
+* [Counter](http://redux.js.org/docs/introduction/Examples.html#counter) ([source](https://github.com/reactjs/redux/tree/master/examples/counter), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/counter))
+* [Todos](http://redux.js.org/docs/introduction/Examples.html#todos) ([source](https://github.com/reactjs/redux/tree/master/examples/todos), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/todos))
+* [Todos with Undo](http://redux.js.org/docs/introduction/Examples.html#todos-with-undo) ([source](https://github.com/reactjs/redux/tree/master/examples/todos-with-undo), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/todos-with-undo))
+* [TodoMVC](http://redux.js.org/docs/introduction/Examples.html#todomvc) ([source](https://github.com/reactjs/redux/tree/master/examples/todomvc), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/todomvc))
+* [Shopping Cart](http://redux.js.org/docs/introduction/Examples.html#shopping-cart) ([source](https://github.com/reactjs/redux/tree/master/examples/shopping-cart), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/shopping-cart))
+* [Tree View](http://redux.js.org/docs/introduction/Examples.html#tree-view) ([source](https://github.com/reactjs/redux/tree/master/examples/tree-view), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/tree-view))
+* [Async](http://redux.js.org/docs/introduction/Examples.html#async) ([source](https://github.com/reactjs/redux/tree/master/examples/async), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/async))
 * [Universal](http://redux.js.org/docs/introduction/Examples.html#universal) ([source](https://github.com/reactjs/redux/tree/master/examples/universal))
-* [Real World](http://redux.js.org/docs/introduction/Examples.html#real-world) ([source](https://github.com/reactjs/redux/tree/master/examples/real-world))
+* [Real World](http://redux.js.org/docs/introduction/Examples.html#real-world) ([source](https://github.com/reactjs/redux/tree/master/examples/real-world), [sandbox](https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/real-world))
 
 If you're new to the NPM ecosystem and have troubles getting a project up and running, or aren't sure where to paste the gist above, check out [simplest-redux-example](https://github.com/jackielii/simplest-redux-example) that uses Redux together with React and Browserify.