Include details of how to update Postman Collections in CONTRIBUTING.md

[ProBeta1]

@ProBeta1 thank you, but is there a way I can edit documentation myself? We evolve this API and I need to reflect changes in the doc. Please advise.

Originally posted by @elitemaks in #111 (comment)

[kinlane]

With the collections owned by a variety of people this isn't possible. I am considering however to create a GitHub for each collection on the list, to help manage where to ask for changes and which ones are within our contorl.