Add comment for reproducible builds

See https://maven.apache.org/guides/mini/guide-reproducible-builds.html

Author: garydgregory

pom.xml

@@ -116,6 +116,7 @@
     <!--Commons Release Plugin -->
     <commons.bc.version>4.4</commons.bc.version>
     <commons.release.isDistModule>true</commons.release.isDistModule>
+    <!-- project.build.outputTimestamp is managed by Maven plugins, see https://maven.apache.org/guides/mini/guide-reproducible-builds.html -->
     <project.build.outputTimestamp>2024-12-18T21:13:07Z</project.build.outputTimestamp>
     <!-- JaCoCo: Don't make code coverage worse than: -->
     <commons.jacoco.haltOnFailure>true</commons.jacoco.haltOnFailure>