Update securedrop-workstation release docs to include yum-test deployment

[zenmonkeykstop]

Current docs don't reflect the release workflow, which includes the usual 1+ RCs, deployment to the yum-test server first, and preflight checks on yum-qa. The docs should be updated to reflect the actual process.

See freedomofpress/securedrop-workstation#920 (comment) for notes taken on the fly during the last release...