Improve README for app discovery

[olujicz]

Summary

• sharpen the opening description around external-monitor control, DDC/CI, and software fallback
• replace the developer-focused badge with release, CI, and Homebrew trust signals
• add an early Homebrew command and a concise “Why Dimmerly?” section
• reduce repeated distribution details while retaining the capability comparison
• add direct changelog, bug-report, and monitor-compatibility links

Why

The README already documented Dimmerly thoroughly, but its first screen could communicate the primary value and installation path more quickly. Distribution differences were also repeated in several sections. This update improves discovery and download conversion while keeping the technical guidance available.

Validation

• just test
• .github/scripts/test-ci-scope.sh
• git diff --check
• rendered the README with GitHub’s Markdown API and verified the new sections and links
• confirmed all external badge, release, workflow, and issue-form URLs return HTTP 200
• confirmed the CI scope classifier returns full_ci=false for this README-only change