Add a RATIONALE.md that explains why we choose to use GraphQL to intermediate gRPC services

[codefromthecrypt]

Many are not familiar with GraphQL and less familiar with why it is useful vs directly using grpc-web or presenting a REST api for UI consumption.

Since these decisions pre-dated OSS, they aren't obvious. Rather than waiting for folks to forget, let's try to capture a RATIONALE.md that overviews why we settled on GraphQL, and what options were dismissed and for what reasons.

[codefromthecrypt]

@aaron-steinfeld I'm assigning this to you as I don't know anyone else that would know the answer to this.