Add comment about source.scm.tag=HEAD warning

Actually it looks like the warning is not actually caused by usage of
`HEAD` as value, but rather because the project has a snapshot version
during development (which is expected though), see
https://github.com/apache/maven-artifact-plugin/blob/maven-artifact-plugin-3.5.0/src/main/java/org/apache/maven/plugins/artifact/buildinfo/BuildInfoWriter.java#L140

But this is not a problem either since during release a non-snapshot
version is used.

pom.xml

@@ -196,6 +196,9 @@
  <executions>
  <execution>
  <goals>
+ <!-- This logs a warning about `source.scm.tag=HEAD`, but this can be ignored;
+ during release Maven Release Plugin temporarily changes the `source.scm.tag`
+ value to the actual Git tag, which will then not cause a warning -->
  <goal>buildinfo</goal>
  </goals>
  </execution>