Update RELEASING doc

Our releasing docs were a bit outdated, missing some things we do for a
release. I updated it based on my experience with the 1.16.0 release,
and also added some tips.

Change-Id: Ia33a44a2a1f526769be195796c3456fb47269863
Reviewed-on: http://gerrit.cloudera.org:8080/18466
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <alexey@apache.org>
1 file changed