Add activemq properties to appendix

Fixes gh-618

Author: Dave Syer

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -161,6 +161,8 @@ content into your application; rather pick only the properties that you need.
 
 	# ACTIVEMQ ({sc-spring-boot-autoconfigure}/jms/ActiveMQProperties.{sc-ext}[ActiveMQProperties])
 	spring.activemq.broker-url=tcp://localhost:61616 # connection URL
+    spring.activemq.user=
+    spring.activemq.password=
 	spring.activemq.in-memory=true
 	spring.activemq.pooled=false