Release playbook

Steps to do when making a new release:

• unless bugfix release: merge master into stable
• update version number and codename in src/shared/version.go, commit+push onto stable with version number as message
• tag this commit with the version number and push the tag
• use util/make-release-tarball.sh to make a release tarball
• write release notes on Github, attach release tarball
• upload new version to AUR (bump version, then updpkgsums && mksrcinfo && makepkg -s && git commit -a && git push; this verifies once more that the release tarball is intact and passes all tests)
• if the documentation has changed: make website && git -C website commit -a && git -C website push
• tweet about the release (with reference to release notes on Github)