Add CONTRIBUTING.md file

[jcubic]

This is file is a standard file used on GitHub when creating a PR and we can link it in README.

The file should say how to create recipes. We will need to decide how to go about it. If we migrate to PR, then we will need to remove or improve the recipe issue template and create a template for PR linked in CONTRIBUTING file. It actually can be the same file since it's already in markdown.

Also, it would be nice to automate publishing the cookbook using CI/CD, we can use free Travis service or GitHub actions.

[lassik]

I would favor using PRs for solutions, with a PR template.

There are some GitHub actions for Scheme but I haven't tested them.