Add apache license badge

Author: hazendaz

README.md

@@ -3,6 +3,7 @@ MYBATIS Data Mapper Framework
 
 [![Build Status](https://travis-ci.org/mybatis/mybatis-3.svg?branch=master)](https://travis-ci.org/mybatis/mybatis-3)
 [![Maven central](https://maven-badges.herokuapp.com/maven-central/org.mybatis/mybatis/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.mybatis/mybatis)
+[![Apache 2](http://img.shields.io/badge/license-Apache%202-red.svg)](http://www.apache.org/licenses/LICENSE-2.0)
 
 ![mybatis](http://mybatis.github.io/images/mybatis-logo.png)