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

[hesreallyhim]

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:

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    "readme": {
      "url": "https://api.github.com/repos/anthropics/skills/contents/README.md",
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