DOCS: Add `CONTRIBUTING.md` and then enforce it

This is a very good resource, and an extremely popular repo. I like it very much.

I don't see any indication that this repo accepts external contributions of skills.

I urge the maintainers @klazuka et al. who set up this repo to add a CONTRIBUTING document stating the repo's policy about contributions, and then to enforce it. It's full of spam, and I see no reason to give platform to such projects. I urge Anthropic to exhibit basic community awareness. This is not something to be proud of, and does not do justice to the quality of the content:

```
{
"health_percentage": 25,
"description": "Public repository for Agent Skills",
"documentation": null,
"files": {
"code_of_conduct": null,
"code_of_conduct_file": null,
"contributing": null,
"issue_template": null,
"pull_request_template": null,
"license": null,
"readme": {
"url": "https://api.github.com/repos/anthropics/skills/contents/README.md",
"html_url": "https://github.com/anthropics/skills/blob/main/README.md"
}
},
"updated_at": null,
"content_reports_enabled": false
}
````