adding a contribution md

windfall-elimination doesn't have a contribution file. To start, it could have the date and time of meeting. It could provide some rules for raising issues and pull requests. Finally, a reference link to the README.md and a reference link to the code of conduct.

Start by reviewing README.md and the code of conduct to understand the project’s existing guidance. Add a contribution markdown file covering meeting date and time, rules for issues and pull requests, and links to both documents; done means all requested sections and references are present.

Written by the indexing model from the issue text.