Clarify README for cbrtester (#193)

Clarify what a "stall" means in the context of the cbrtester.

Author: FR4NK-W

cbrtester/README.md

@@ -2,5 +2,7 @@
 
 Sends at a constant rate using the specified path.
 
-The client chooses the packet size, the rate and the path, and the server
-prints out the receive rate and stalls (freezes) if any are detected.
+The client chooses the packet size, the rate and the path.
+The server prints out the receive rate and stall events (freezes or
+interruptions in the received packet stream of more than 100ms) if any are
+detected.