add reason for suppression

Author: littleaj

agent/dependency-check-suppressions.xml

@@ -3,6 +3,7 @@
     <suppress>
         <notes><![CDATA[
       file name: instrumentation-logback-0.14.2.jar
+      This is logback instrumentation, not the logback library itself.
       ]]></notes>
         <packageUrl regex="true">^pkg:maven/org\.glowroot\.instrumentation/instrumentation\-logback@.*$</packageUrl>
         <cve>CVE-2017-5929</cve>