Add Milestoner (Ruby Developer Experience)

[bkuhlmann]

Why

Hello. 👋 This is in response to the upcoming Ruby Developer Experience newsletter in order to share more tooling that'll enrich people's lives.

I'm the author of the Milestoner gem and use it to automate the release note generation and automatic versioning of every project I work on. I've written about Milestones and spoken about Milestones many times in the past and think sharing this with folks will definitely level up their game if you'd like strong tooling for additional automation of your workflow.