Add Tufts University capstone course sponsorship guidelines

Why is this change being made?

We may continue to volunteer as sponsors for the Tufts University capstone project in the future. Regardless, I think we could use capstone courses as part of learning and development for UX team members, as it can help us build mentoring and coaching skills, which are crucial in our positions. I've started by summarizing the project itself, what PDs/Researchers/TWs can get out of it, and best practices from previous experience for those who may want to volunteer in the future.

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