Official Use-case for CovidCreds.com

[dlandi]

@redth Thanks so much for this work. In preparation to use this repo for the http://CovidCreds.com initiative, I have written a use case for it. Please check it out here: https://docs.google.com/document/d/1l2gfwGZy1HlJB6FmphI_xZAPSAuvrN1fdAaI7uNhAio/edit?usp=sharing and please comment. I would love your feedback.

Perhaps you can post a reference to the use case on the project ReadMe file, and invite other developers to contribute.

Thanks.