Document post-release reproducibility verification for IUKs

Originally created by @intrigeri on #16052 (Redmine)

As explained on #12629 (comment 44389), the draft doc we have currently feels too complex, error-prone, and work-hungry.

Edited May 15, 2020 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information