Added and documented Spring Boot BOM

Folks shouldn't use main Fabric8 BOM in their Spring Boot microservices. For example we use Spring 3.x in Fabric8, but Spring Boot users should use Spring 4.x.

My Fabric8 Spring Boot BOM imports regular Spring Boot BOM, so people can just import my BOM and enjoy the "blessed" versions of the both projects.

More details on usage are documented in MD file.