Bring in Danger: https://danger.systems/ruby/

[murny]

Might be a fun beginner issue. We use Danger in pushmipullyu, it might be nice to add it to jupiter (and maybe other repos?)
Can help us with things like:

• remind us if we forgot to add a CHANGELOG entry which happens alot!
• adding tests if lots of code were being added
• break up big PRs
• rebase PRs if they have a ton of commits
• adding detail descriptions etc.

A few different dangerfile examples in the wild:
https://github.com/discourse/discourse/blob/master/Dangerfile
https://github.com/capistrano/danger/blob/master/Dangerfile

and ours from pushmipullyu:
https://github.com/ualbertalib/pushmi_pullyu/blob/master/Dangerfile

And the PR for adding this to pushmipullyu: ualbertalib/pushmi_pullyu#61