Add accessibility optimization link to README (#40)

Added link for [optimizing GitHub Copilot for
accessibility](https://accessibility.github.com/documentation/guide/copilot-instructions/).

Author: cehfisher

README.md

@@ -112,7 +112,7 @@ The a11y scanner leverages Copilot coding agent, which can be configured with cu
 - Repository-wide: `.github/copilot-instructions.md`
 - Directory/file-scoped: `.github/instructions/*.instructions.md`
 
-📚 [Adding repository custom instructions](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions) | [Copilot .instructions.md support](https://github.blog/changelog/2025-07-23-github-copilot-coding-agent-now-supports-instructions-md-custom-instructions/) | [Copilot agents.md support](https://github.blog/changelog/2025-08-28-copilot-coding-agent-now-supports-agents-md-custom-instructions)
+📚 [Adding repository custom instructions](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions) | [Copilot .instructions.md support](https://github.blog/changelog/2025-07-23-github-copilot-coding-agent-now-supports-instructions-md-custom-instructions/) | [Copilot agents.md support](https://github.blog/changelog/2025-08-28-copilot-coding-agent-now-supports-agents-md-custom-instructions) | [Optimizing GitHub Copilot for accessibility](https://accessibility.github.com/documentation/guide/copilot-instructions)
 
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