docs: guidelines for merging a new car port

[jyoung8607]

This is an attempt to codify some oral history around requirements to merge a car port. Very, very much a draft.

• [ ] Bikeshedding of basically every line in this document
• [ ] Bikeshed what repo it belongs in
• [ ] Cleanup and organization
• [ ] Finish some TODO items
• [ ] Users should know their test routes will become public
• [ ] Cars in release should never use an API that isn't used by the manufacturer (see: Toyota LKA vs LTA)

[github-actions[bot]]

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