Add entry for gnomad graphql

[cmungall]

http://gnomad-api.broadinstitute.org/

Note that we don't have a strategy for graphql yet...

[stevencox]

Also green services as GraphQL.

[stevencox]

Here's a sketch of an approach. If this looks reasonable maybe we can make some progress at the Hackathon. Feedback welcome.

[cmungall]

I like the approach above

There is also this approach from @micheldumontier for registering a simple stub:
SmartAPI/smartapi_registry#3 (comment)