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]

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