Add comment for reproducible builds

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

Author: garydgregory

pom.xml

@@ -182,6 +182,7 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
+    <!-- project.build.outputTimestamp is managed by Maven plugins, see https://maven.apache.org/guides/mini/guide-reproducible-builds.html -->
     <project.build.outputTimestamp>2024-02-01T03:27:02Z</project.build.outputTimestamp>
     <maven.compiler.source>17</maven.compiler.source>
     <maven.compiler.target>17</maven.compiler.target>