fix: Mention CLA in CONTRIBUTING.md

[u9000-Nine]

Closes: #2146

Description

Briefly mention CLA in the legal section of CONTRIBUTING.md

Fixes #2146

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[u9000-Nine]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

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