add doc on additional worker information

Overall-architecture.md

@@ -95,6 +95,15 @@ Commands from worker to broker:
 - **init** - introduce yourself to the broker. Useful on startup or after reestablishing lost connection. Requires at least 2 arguments:
     - `hwgroup` - hardware group of this worker
     - `header` - additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers.
+
+    There is also an optional third argument - additional information. If 
+    present, it should be separated from the headers with an empty frame. The 
+    format is the same. Supported keys for additional information are:
+    - `description` - a human readable description of the worker for 
+      administrators (it will show up in broker logs)
+    - `current_job` - an identifier of a job the worker is now processing. This 
+      is useful when we're reassembling a connection to the broker and need it 
+      to know the worker won't accept a new job.
 - **done** - job evaluation finished, see **done** command in [Main communication](#main-communication) section.
 - **progress** - evaluation progress report, see **progress** command in [Progress callback](#progress-callback) section
 - **ping** - tell broker I'm alive, no arguments