Add Gitter badge (#502)

Author: gitter-badger

README.adoc

@@ -1,5 +1,7 @@
 = Spring Boot Migrator (SBM)
 
+image:https://badges.gitter.im/spring-boot-migrator/community.svg[link="https://gitter.im/spring-boot-migrator/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"]
+
 [quote]
 ____
 Spring Boot Migrator (SBM) aims to help developers upgrade or migrate to https://spring.io/projects/spring-boot[Spring Boot] by providing recipes for automated migrations.