Update CONTRIBUTING.md

[johndawsonneufeld]

add legal blurb for CLA w/ outside OZ account

Documentation Pull Request

Summary

Type of Change

• New documentation
• Documentation update/revision
• Fix typos or grammar
• Restructure/reorganize content
• Add examples or tutorials
• Update API documentation
• Other: ___________

Related Issues

Fixes #
Relates to #

Checklist

• Build succeeds locally with pnpm run build
• Lint is successful when running pnpm run check
• Docs follow OpenZeppelin Documentation Standards

Additional Notes

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[github-actions]

Thank you for your contribution to OpenZeppelin Docs. Before being able to integrate those changes, we would like you to sign our Contributor License Agreement. You can sign the CLA by just posting a Pull Request Comment with the sentence below. Thanks.

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I confirm that I have read and hereby agree to the OpenZeppelin Contributor License Agreement

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_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}