documentation: make contributing doc really clear

[leondz]

step-by-step guide

• [ ] soft intro
• [ ] structure
  • [ ] talk about how we manage the project - on github; get in touch!; open license only
  • [ ] talk about project scope
• [ ] garak's code
  • [ ] talk about general program structure
  • [ ] point to docs for how the parts work
• [ ] getting started coding
  • [ ] set up a gh account, fork garak main repo
  • [ ] work out what you're doing. if it's from a good first issue, drop a note
  • [ ] give pointers to good classes to look at
  • [ ] code it up
  • [ ] (refer to existing content on testing)
  • [ ] run black!
  • [ ] write your own tests
  • [ ] when you're done, send a pull request. we'll discuss the code and hopefully merge it in, maybe with some edits!
• [ ] big thanks; where to find us; look forward to welcoming your code in garak