Add AI Contribution Policy

[florence-bockting]

Submission Checklist

• [ ] Run unit tests: ./runTests.py src/test/unit
• [ ] Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

• Adds a reference to the Stan AI Contribution Policy (see Stan-wiki: https://github.com/stan-dev/stan/wiki/AI-Contribution-Policy) to the CONTRIBUTING.md so contributors are aware of it before submitting PRs.
• Fix link to Zenodo badge in README.md

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Aalto university

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)