dev docs: rackup doesn't seem to get installed

[sersorrel]

Describe the error
Following the development setup instructions, I don't understand where rackup is meant to get installed. I bundle installd, and bundle exec rake works fine, but neither rackup nor bundle exec rackup work.

Affected document
README.md, docs/DevEnvironment.md

Additional context
Your documentation is honestly better than 90% of projects I've interacted with :) I just happen to be unfamiliar with Ruby and am therefore tripping over every single crack in the floor