communication (broker - frontend) - ack

Overall-architecture.md

@@ -186,6 +186,7 @@ Commands from frontend to broker:
 
 Commands from broker to frontend (all are responses to **eval** command):
 
+- **ack** - this is first message which is sent back to frontend right after eval command arrives, basically it means "Hi, I am all right and am capable of receiving job requests", after sending this broker will try to find acceptable worker for arrived request
 - **accept** - broker is capable of routing request to a worker
 - **reject** - broker can't handle this job (for example when the requirements 
   specified by the headers cannot be met). There are (rare) cases when the