Not sure where to send feedback about this, but i think an extra line is needed for the “What administrators need to do” section in the release notes (SecureDrop 0.6 Released).
The line missing is:
git fetch --tags
(mentioned here Add "securedrop-admin upgrade" command · Issue #2976 · freedomofpress/securedrop · GitHub)
Otherwise you get an error when trying to checkout 0.6
error: pathspec ‘0.6’ did not match any file(s) known to git.