logback.xml: Adding logging for the Spring Cloud Config client

So that you see your application printing which Config Server it is
connecting to.

2017-04-13 23:46:14 [main] INFO
o.s.c.c.c.ConfigServicePropertySourceLocator - Fetching config from
server at: http://localhost:8888

src/main/resources/logback.xml

@@ -15,6 +15,11 @@
 		additivity="false">
 		<appender-ref ref="STDOUT" />
 	</logger>
+
+  <logger name="org.springframework.cloud.config.client" level="debug"
+    additivity="false">
+    <appender-ref ref="STDOUT" />
+  </logger>
 
 	<root level="error">
 		<appender-ref ref="STDOUT" />