Update changelog for 1.0+beta12 release
Update changelog for 1.0+beta12 release

janestreet/jbuilder,janestreet/jbuilder,rdavison/jbuilder,dra27/jbuilder,rdavison/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder