Contributing docs: add some first-time contributing hints

[ddbeck]

Fixes #1600

[petele]

I tried adding Prettier to VSCode, and it didn't report a warning I expected, but it was reported when running npm test.

Repro steps:
Put 2 spaces after description:.

Expect: ✅ Error in VS Code reporting extra space, ✅ npm run test reports warning
Actual: ❌ No error in VS Code, ✅ npm run test reports warning

[ddbeck]

@petele I don't think the Prettier VS Code extension gives warnings. I think it's only a formatter. You can invoke it via the Command Palette "Format Document" or (my recommendation) turn on the editor.formatOnSave preference. I also suggest turning on the prettier.requireConfig setting so it doesn't start running everywhere, unbidden.