DOC: Add contributors to 0.12.0 release notes
DOC: Add contributors to 0.12.0 release notes

ibis-project/ibis,ibis-project/ibis,cloudera/ibis,cloudera/ibis,cpcloud/ibis,cloudera/ibis,deepfield/ibis,ibis-project/ibis,deepfield/ibis,deepfield/ibis,cpcloud/ibis,cpcloud/ibis,cpcloud/ibis,deepfield/ibis,ibis-project/ibis