headers elaboration

Broker.md

@@ -30,9 +30,16 @@ especially when some kind of error happens on the server.
 The `worker_registry` class is used to store information about workers, their 
 status and the jobs in their queue. It can look up a worker using the headers 
 received with a request (a worker is considered suitable if and only if it 
-satisfies all the headers). It also uses a basic load balancing algorithm -- the 
+satisfies all the headers). The headers are arbitrary key-value pairs, which 
+are checked for equality by the broker. However, some headers require special 
+handling, namely `threads`, for which we check if the value in the request is 
+lesser than or equal to the value advertised by the worker, and `hwgroup`, for
+which we support requesting one of multiple hardware groups by listing multiple
+names separated with a `|` symbol (e.g. `group_1|group_2|group_3`.
+
+The registry also implements a basic load balancing algorithm -- the 
 workers are contained in a queue and whenever one of them receives a job, it is 
-moved to the back, which makes it less likely to receive another job soon.
+moved to its end, which makes it less likely to receive another job soon.
 
 When a worker is assigned a job, it will not be assigned another one until we 
 receive a `done` message from it.