Add NOTICE.txt file to Impala repo

Change-Id: Ic1a1304d7425e4bc56daebf4418045889410d6a8
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3227
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Lenni Kuff <lskuff@cloudera.com>