Add repo-specific CONTRIBUTING.md clarifying external contribution policy

[echarrod]

I wanted to work on #688 (respect $SHELL for command execution), but this repository doesn’t appear to contain the Copilot CLI runtime/source code needed to implement it—only release/docs assets like README, changelog, and install.sh.

The “Contributing” tab currently points to the org-level CONTRIBUTING.md inherited from https://github.com/github/.github/blob/main/CONTRIBUTING.md, which doesn’t describe this repo’s contribution reality.

Could you add a repository-local CONTRIBUTING.md (or a prominent README section) that explicitly states whether external code contributions are accepted, and if not, that PRs from outside GitHub can’t be merged here; also clarify what kinds of contributions/feedback are welcome (issues, docs changes to this repo, feature requests)?