add logback.xml

_code-samples/credential/java/logback.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Quiets xrpl4j's DEBUG chatter so tutorial output stays readable.
+     Raise xrpl4j to DEBUG to see wire-level transaction details. -->
+<configuration>
+  <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
+    <encoder>
+      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -- %msg%n</pattern>
+    </encoder>
+  </appender>
+
+  <logger name="org.xrpl.xrpl4j" level="WARN"/>
+
+  <root level="INFO">
+    <appender-ref ref="CONSOLE"/>
+  </root>
+</configuration>