add note about included com.google.gerrit.extensions sources
add note about included com.google.gerrit.extensions sources

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