This diff lays the groundwork for release automation (T646) by adding a new script release.sh.
The current iteration of release.sh doesn't do much on its own. For now, it generates
a release commit containing only updates to the manpages and tags that commit. In the future,
it will do many of the automatible steps in doc/release-process.md (see TODOs in the script).
While release.sh is intended to be run by a release bot (currently WIP), it can be run manually as well.