Add license badge

Author: joshiste

README.md

@@ -1,5 +1,6 @@
 spring-boot-admin
 =================
+[![Apache License 2](https://img.shields.io/badge/license-ASF2-blue.svg)](https://www.apache.org/licenses/LICENSE-2.0.txt)
 [![Build Status](https://travis-ci.org/codecentric/spring-boot-admin.svg)](https://travis-ci.org/codecentric/spring-boot-admin)
 [![Coverage Status](https://coveralls.io/repos/codecentric/spring-boot-admin/badge.svg)](https://coveralls.io/r/codecentric/spring-boot-admin)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/de.codecentric/spring-boot-admin/badge.svg)](https://maven-badges.herokuapp.com/maven-central/de.codecentric/spring-boot-admin/)