Added new chapter

ReactForBeginners/Videos.md

@@ -139,3 +139,24 @@ class MyClass extends React.Component {
 	}
 }
 ```
+
+## Video 12: 
+* Docs: https://github.com/ReactTraining/react-router/tree/master/docs
+* Use context types:
+```javascript
+// MyComponent.js
+MyComponent.contextTypes = {
+	router:  React.PropTypes.object
+}
+```
+* This gives access to these functions:
+    - blockTransitions
+    - createHref
+    - replaceWith
+    - transitionTo
+* Usage:
+```javascript
+this.context.router.transitionTo('...')
+```
+
+## Video 13: Understanding State