Add comment for reproducible builds

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

Author: garydgregory

pom.xml

@@ -58,6 +58,7 @@
     <!-- Commons Release Plugin -->
     <commons.bc.version>2.0.0-SNAPSHOT</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>2025-01-07T14:08:08Z</project.build.outputTimestamp>
     <!-- JaCoCo: Don't make code coverage worse than: -->
     <commons.jacoco.haltOnFailure>true</commons.jacoco.haltOnFailure>