docs: carve out a no-visual-delta exception for screenshots

Keeps the before/after-screenshot rule for UI changes but lets PRs with
no visual delta (refactors, accessibility labels, focus order) state
that in the description instead of attaching meaningless images.

Lands the maintainer-suggested wording from #36.

Co-authored-by: Alvie Stoddard <alviestoddard@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 3 people

CONTRIBUTING.md

@@ -37,13 +37,13 @@ open packaging/Pesty.app
 - Keep it dependency-free. Prefer system frameworks (AppKit, SwiftUI, Carbon, ServiceManagement).
 - Match the existing style. Small, focused changes; no unrelated refactors in the same PR.
 - Test on both Apple Silicon and Intel where it matters (the release is universal).
-- UI changes: include a before/after screenshot of the strip.
+- UI changes: include a before/after screenshot of the strip. If the change has no visual delta (refactor, accessibility labels, focus order), say so in the PR description instead.
 
 ## Pull requests
 
 1. Fork and branch from `main`.
 2. Make your change; ensure `swift build` is clean (no warnings).
-3. Open a PR with a clear description and screenshots for UI changes.
+3. Open a PR with a clear description and, for UI changes, before/after screenshots of the strip.
 
 ## Good first issues