Contribution guidelines for examples, with an example example

[gfscott]

This PR lays out a draft model for examples, as well as some initial documentation for contributing examples.

Notes:

• I copied in a subset of @juanpprieto's Multipass example repo verbatim. It's a reasonably complex example which is why we wanted to start with it. I thought it was important that the file links in the README worked as intended to give a full sense of how it feels.
• However, JP's original repo covers two cases: persisting a session through to checkout, and authenticating with Google Oauth. I dropped the third-party auth thing here. I think two separate examples might be better? Or even three — two for each separate use case, and a third showing the combo. Things to discuss on the tradeoffs there.
• We should nail down more specifics on the functional baseline for examples — as in, should each example folder have its own package.json file, its own remix config, a server.ts file, etc. I think our long-term intent is to make it easy to run these locally, so what's the minimum set of files that gets us there without becoming a maintenance headache?

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[benjaminsehl]

This is awesome, thank you guys!

[benjaminsehl]

In addition to the comment on the readme — want to confirm that our multipass is living up to our expectations around what's stated in the contributing guide cc: @juanpprieto