✨ Add check writing guidance

Brie Carranzarequested to merge
bcarranza-readme-for-checks-contributing-for-the-project-itself into main

💯 This MR is in support of our goal to get to 100 checks -- #27. This MR includes information I think will be helpful to other contributors based on my experience contributing a couple checks and chatting with others.

✨ This MR adds check writing guidance as discussed during gitlab-com/support/support-pairing#16042 and described briefly in !50 (merged).

📋 TODOs

Prior to merging...

  • Decide how to handle one-time setup info
  • Decide what goes in README.md, what goes in CONTRIBUTING.md and how to handle info that applies to both
    • All in one.
Edited by Brie Carranza

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