content/contributing-to-a-lesson: landing page for contributors

[rkdarst]

• If we want to use this widely, we need to explain to others how to contribute to one of these lessons. This is designed to be that page.
• I have avoided repeating things that are explained other places (no time to re-explain), though eventually these points could be elaborated (example: how to make a pull request).
• Review:
  • This is a quick pass, it is worth reading it carefully and to see what is wrong or can be simplified. Though, I am sure it will be continually improved over time.