"How to write code excerpts" in the readme

[MaryaBelanger]

I feel like I explain the mechanics of code excerpts to contributors (both internal and external) quite a bit in PR reviews. Would it make sense to add a brief section that explains how to edit or write a new code excerpt (like, where the code goes and how to connect it to a code block on a md page) in the readme @parlough (since you're the readme author)? Maybe right before the refresh code excerpts section.

If that sgty, I'll add it in!

[parlough]

I think this would be great to add. Either in the README or a section of the https://github.com/dart-lang/site-www/blob/main/CONTRIBUTING.md doc that the README could link to. Whichever you prefer!