Update README.md

[jadmadi]

Summary

Adds an installation section to the README.md using npx skills add slavingia/skills for developers using other AI agents compatible with skills.sh.

Changes

• Added a dedicated For other agents using skills.sh section under ## Installation.
• Kept existing Claude Code and local clone installation steps intact.

Motivation

Allows users who leverage skills.sh-compatible agents outside of Claude Code to easily discover and install these skills via the CLI package runner.

[test-confidence]

⏳ Test Confidence: 0.0%

░░░░░░░░░░░░░░░░░░░░ 0.0%
0 tests passed

Waves
⏳ 80% — 0 tests — Change is documentation-only (README.md addition of installation instructions for skills.sh). No code paths affected, no unit tests applicable.
⬜ 95% — 0 tests — Documentation change has no runtime behavior to integration test.
⬜ 99% — 0 tests — No feature specs exist in this repo (skills marketplace, not a Rails app) and no E2E flows touch README content.
⬜ 100% — ~0 remaining — Full suite for mathematical certainty, though no tests exist in the tree.

RUNNING — Wave 1 of 4