Add link to CLA to GOVERNANCE.md

[yuvipanda]

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I absolutely love the GOVERNANCE.md file! It's excellent and I wish more projects had that. Thank you for writing it!

My understanding from looking at various pull requests is that a CLA (https://marimo.io/cla) with standard copyright assignment to Marimo Inc. Could this be included in GOVERNANCE.md as well? I think CLAs form an important part of open source project governance, and it would be very useful to have that be explicitly mentioned there.

Regardless, wonderful project and I'm happy to see all the progress y'all are making!