Api principles (preliminary, very rough draft)

I agree with @krisleech https://github.com/HouseTrip/guidelines/pull/20/files#r16402442 it is more explicit and you don't always have to look into header what other information an api returns, particularly useful when a PM is testing things.

I think we need to be very mindful about what to keep in header and IMHO we should not transmit any actual data in the header. Any variable parts of the resources that the consumer needs to work with or on the basis of which the api response changes should be part of the response. Opinions?