Add link to C++ api docs placeholder

Author: smokestacklightnin

tensorflow_serving/g3doc/index.md

@@ -7,5 +7,5 @@ TensorFlow Serving is a flexible, high-performance serving system for machine le
 Detailed developer documentation on TensorFlow Serving is available:
 
 - [Architecture Overview](../architecture)
-- Server API
+- [Server API](../../api/tensorflow_serving)
 - [REST Client API](../../api/api_rest)