docs: add CONTRIBUTING.md to fix broken link in PR workflow

[maayanmatsliah-tech]

Hi! This PR fixes issue #1067.
The PR workflow template referenced a CONTRIBUTING.md file that didn't exist, so I added it to the root of the repo. The file points to the Conda contributing guide and the code of conduct.
I'm happy to make any changes if necessary. Hope this helps!

[conda-bot]

We require contributors to sign our Contributor License Agreement and we don't have one on file for @maayanmatsliah-tech.

In order for us to review and merge your code, please e-sign the Contributor License Agreement PDF. We then need to manually verify your signature, merge the PR (conda/infrastructure#1330), and ping the bot to refresh the PR.

[marcoesters]

Thank you for your PR! Please add a news file as outlined in the PR template checklist. Also please let me know when you signed the CLA and I can review the PR.

[maayanmatsliah-tech]

Hi! I've added the news file. I also signed the CLA. Please let me know if there is anything else I can do!

[marcoesters]

We will have to wait for the CLA to be reviewed by the conda/infrastructure team (I personally cannot do that). Once that's done, we can continue with this PR.