Create CONTIBUTING file

[mariamedp]

We should have a CONTRIBUTING.md file to explain how to contribute to the repo, and remove the section from the README. Example: https://github.com/microsoft/solution-accelerator-many-models/blob/master/CONTRIBUTING.md

It'd be nice to have detailed instructions (or links to official instructions, depending on the case) on how to:

• [ ] Fork the repo or create a branch
• [ ] Submit a bug
• [ ] Create a pull request
• [ ] Set up a test environment and pipelines so contributors can make sure everything works before submitting the PR.

To be adapted from the Contribution Guide that @FlorianPydde created in the internal wiki.