add CONTRIBUTING file #22

Add contribution guidelines via a CONTRIBUTING file which covers the following:

• Contributors should review the Songcamp welcome channel and be familiar with the CODE OF CONDUCT.
• Contributors should discuss potential contributions via the cc0-main/CC0lab channel in the Songcamp discord to gauge interest, receive feedback, and coordinate work.
• All contributions must be public domain and each contributor must have a CC0 waiver on file.

The last point requires some thought/research. Here are some useful references to get started:

• https://www.sqlite.org/copyright.html
• https://transbank.info/cc0/
• https://contributoragreements.org/
• https://eips.ethereum.org/EIPS/eip-1#what-belongs-in-a-successful-eip
• https://opensource.microsoft.com/pdf/microsoft-contribution-license-agreement.pdf
• https://github.com/cloud-gov/pages-core/blob/staging/CONTRIBUTING.md#public-domain