Update README.md

Author: johnkil

README.md

@@ -3,6 +3,7 @@ Android-MultiBackStack
 
 [![Android Arsenal](https://img.shields.io/badge/Android%20Arsenal-MultiBackStack-brightgreen.svg?style=flat)](http://android-arsenal.com/details/1/3636)
 [![JitPack](https://jitpack.io/v/jetradarmobile/multibackstack.svg)](https://jitpack.io/#jetradarmobile/multibackstack)
+[![API](https://img.shields.io/badge/API-9%2B-brightgreen.svg?style=flat)](https://android-arsenal.com/api?level=9)
 [![TravisCI](https://travis-ci.org/JetradarMobile/android-multibackstack.svg?branch=master)](https://travis-ci.org/JetradarMobile/android-multibackstack)
 [![AndroidDev Digest](https://img.shields.io/badge/AndroidDev%20Digest-%2396-blue.svg)](https://www.androiddevdigest.com/digest-96/)