Updated release guide.
Updated release guide.

Updated the release guide in `docs/release-guide.md` to match
the de-facto process we're using to release new versions.

Also mentioned some additional third-party tooling to the post-release
desription.

Review: https://reviews.apache.org/r/70528/

abudnik/mesos,chhsia0/mesos,kaysoky/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,reneploetz/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,reneploetz/mesos