Service invocation conceptual doc (#66)

* Initial draft

* Initial draft

* Missed initial changes

* Clarification

* Review comment - rephrase

concepts/service-invocation/service-invocation.md

@@ -1,3 +1,17 @@
-# documentation 
+# Service Invocation 
 
-Content for this file to be added
+Dapr-enabled apps can communicate with each other through well-known endpoints in the form of http or gRPC messages.
+
+![Service Invocation Diagram](../../images/service-invocation.png)
+
+
+1. Service A makes a http/gRPC call meant for Service B.  The call goes to the local Dapr sidecar.
+2. Dapr discovers Service B's location and forwards the message to Service B's Dapr sidecar
+3. Service B's Dapr sidecar forwards the request to Service B.  Service B performs its corresponding business logic.
+4. Service B sends a response for Service A.  The response goes to Service B's sidecar.
+5. Dapr forwards the response to Service A's Dapr sidecar.
+6. Service A receives the response.
+
+For more information, see:
+- The [Service Invocation Spec](https://github.com/dapr/spec/blob/master/service_invocation.md)
+- A [HowTo]() on Service Invocation