update README
update README

- add note about supported authentication-methods
- add note about dependencies

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