Add pointers to GRPC proto specification and Triton extensions (kubef…

…low#783)

docs/predict-api/v2/README.md

@@ -8,11 +8,17 @@ standardized around this protocol.
 
 The protocol is composed of a required set of APIs that must be
 implemented by a compliant server. This required set of APIs is
-described in [required_api.md](./required_api.md).
+described in [required_api.md](./required_api.md). The [GRPC proto
+specification](https://github.com/NVIDIA/triton-inference-server/blob/master/docs/protocol/grpc_core_service.proto)
+for the required APIs is available.
 
 The protocol supports an extension mechanism as a required part of the
 API, but no specific extensions are required to be implemented by a
-compliant server.
+compliant server. Inference servers that have implemented extensions
+and the link to those extensions are:
+
+- Triton Inference Server:
+  https://github.com/NVIDIA/triton-inference-server/tree/master/docs/protocol
 
 The protocol is not yet finalized and so feedback is welcome. To
 provide feedback open an issue and prepend the title with "[Predict