CONTRIBUTING: instructions for main results table

[bennn]

CONTRIBUTING.md needs instructions for updating the main table of results in the top-level README.md

Ideas:

• Say how to run _benchmark.rkt and show expected output.
  • At a minimum, pull requests can leave a comment with their output. We can manually add that output to the full table.
  • Optionally, pull request authors can edit the full table manually.
  • Is there a good markdown editor? We could link to that.
  • Link to instructions for installing Racket ... I guess just download.racket-lang.org
• Explain how to run main.rkt Say there's a Docker, explain how to add your new language to it. (This is a lot of work, but should be documented.)
• Explain how to install languages and run main.rkt locally. This is probably not recommended. We should say that clearly.