Document danielvladco.protobuf.graphql.required since proto3 fields all "optional" but GraphQL has "required" concept

[perrydunn]

In proto3 all fields are optional. In GraphQL fields can be required. There is a protobuf field annotation already available and being used within protoc-gen-gql, danielvladco.protobuf.graphql.required, but it does not appear in any of the examples and it is not otherwise documented.

I think it's worth adding the annotation to an example or otherwise documenting its existence.