Update CONTRIBUTING.md

[evanlinde]

This fixes a broken link in CONTRIBUTING.md and removes the link to tmpnb.org (no longer operated by the jupyter project) in README.md.

[welcome[bot]]

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[welcome[bot]]

Congrats on your first merged pull request in this project! :tada: Thank you for contributing, we are very proud of you! :heart:

[consideRatio]

Thank you @evanlinde!