Clarify the content of data in binary frames

Author: jcoglan

README.md

@@ -238,7 +238,8 @@ Both the server- and client-side `WebSocket` objects support the following API:
   Event has no attributes.
 - **`on(:message) { |event| }`** fires when the socket receives a message. Event
   has one attribute, **`data`**, which is either a `String` (for text frames) or
-  an `Array` of byte-sized integers (for binary frames).
+  an `Array` of unsigned integers, i.e. integers in the range `0..255` (for
+  binary frames).
 - **`on(:error) { |event| }`** fires when there is a protocol error due to bad
   data sent by the other peer. This event is purely informational, you do not
   need to implement error recovery.