Publish to RubyGems

[LukasNiessen]

Get a first version published, even a 0.0.1 placeholder, so the name is reserved and the install path can be documented and tested.

[TristanKruse]

Download tracking plan: after the first RubyGems release, use the RubyGems gem page/API for lifetime and per-version totals, ClickGems for PePy-like time-series/version/platform analytics, and the Shields.io gem total-download badge in the README. The package name archunit currently has no RubyGems release, so there is no meaningful download series yet. No publication was attempted here; release credentials/versioning remain part of this issue.