add note about maven artifacts to README
add note about maven artifacts to README

sstafford/gerrit-rest-java-client,sstafford/gerrit-rest-java-client,uwolfer/gerrit-rest-java-client,vogellacompany/gerrit-rest-java-client,uwolfer/gerrit-rest-java-client,vogellacompany/gerrit-rest-java-client