Document how to make a submission

[pinobatch]

In the NESdev Discord server, avivace and I were brainstorming an outline of things to include in README.md:

• How to make a new entry
  • Clone the repo
  • Fill out the most common properties in the schema for game.json
  • Create a valid slug that you're willing to commit to
  • Optionally calculate file hashes
  • Further reading: How to read a JSON schema
• How to run tests before filing a PR
  • Make and activate a Python venv, which is required for installing packages through pip on most *n?x systems since PEP 668
  • Install the required libraries into the venv
  • Run the tests in the tests/ folder; no news is good news
  • Prevent the venv from getting checked into the repository