create annotated tag for v0.19.0

To facilitate setting up CI for mesos repository, we should create annotated tag at the beginning of each release.

This is follow up to http://www.mail-archive.com/dev@mesos.apache.org/msg10915.html

Can you,

a) create one based on this hash 99985d27857fb5a10b26ded8da1a36100780d18b, wherein master was pointed to 0.19.0 release?
b) document the step to create annotated tag at beginning of every release
c) document the step to create lightweight tag for every RC release