Add release notes for 1.16
Add release notes for 1.16

Closes #417

orhankislal/madlib,madlib/madlib,apache/incubator-madlib,orhankislal/incubator-madlib,orhankislal/madlib,apache/incubator-madlib,njayaram2/incubator-madlib,orhankislal/incubator-madlib,njayaram2/madlib,njayaram2/madlib,njayaram2/madlib,orhankislal/madlib,njayaram2/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib,apache/incubator-madlib,njayaram2/incubator-madlib,orhankislal/incubator-madlib,njayaram2/incubator-madlib,orhankislal/madlib,apache/incubator-madlib,njayaram2/madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,madlib/madlib,madlib/madlib,madlib/madlib,madlib/madlib,apache/incubator-madlib,hpandeycodeit/incubator-madlib,orhankislal/madlib,njayaram2/incubator-madlib