Added comment to additional plugin repository explaining why is it there

Signed-off-by: David Matějček <david.matejcek@omnifish.ee>

test-framework/maven/container-runner-maven-plugin/pom.xml

diff --git a/test-framework/maven/container-runner-maven-plugin/pom.xml b/test-framework/maven/container-runner-maven-plugin/pom.xml
index 1034daa..be500e2 100644
--- a/test-framework/maven/container-runner-maven-plugin/pom.xml
+++ b/test-framework/maven/container-runner-maven-plugin/pom.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
-
+    Copyright 2026 Contributors to the Eclipse Foundation.
     Copyright (c) 2015, 2025 Oracle and/or its affiliates. All rights reserved.
 
     This program and the accompanying materials are made available under the
@@ -295,6 +295,7 @@
     </build>
 
     <pluginRepositories>
+        <!-- This is because the groovy-compiler is several versions behind on Maven Central -->
         <pluginRepository>
             <id>groovy-plugins-release</id>
             <url>https://groovy.jfrog.io/artifactory/plugins-release</url>