Updated CHANGELOG for 0.26.0-rc2.
Updated CHANGELOG for 0.26.0-rc2.

Review: https://reviews.apache.org/r/40777

andschwa/mesos,zmalik/mesos,zmalik/mesos,reneploetz/mesos,asamerh4/mesos,gsantovena/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,yuquanshan/customizedMesos,andschwa/mesos,yuquanshan/customizedMesos,neilconway/mesos,chhsia0/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,verizonlabs/mesos,zmalik/mesos,neilconway/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,neilconway/mesos,jpeach/mesos,zmalik/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,verizonlabs/mesos,asamerh4/mesos,verizonlabs/mesos,neilconway/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,abudnik/mesos,gsantovena/mesos,dforsyth/mesos,craimbert/mesos,andschwa/mesos,chhsia0/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,andschwa/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,asamerh4/mesos,andschwa/mesos,craimbert/mesos,neilconway/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,jpeach/mesos,dforsyth/mesos,yuquanshan/customizedMesos,kaysoky/mesos,chhsia0/mesos,andschwa/mesos,neilconway/mesos,reneploetz/mesos,asamerh4/mesos,abudnik/mesos,yuquanshan/customizedMesos,dforsyth/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,craimbert/mesos,craimbert/mesos,craimbert/mesos,abudnik/mesos,zmalik/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,verizonlabs/mesos,gsantovena/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,zmalik/mesos,neilconway/mesos,kaysoky/mesos,reneploetz/mesos,zmalik/mesos,craimbert/mesos,Gilbert88/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,chhsia0/mesos,asamerh4/mesos,asamerh4/mesos,verizonlabs/mesos,asamerh4/mesos,yuquanshan/customizedMesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos