Document why NetConn is included in the library.

netconn.go

@@ -12,6 +12,11 @@ import (
 
 // NetConn converts a *websocket.Conn into a net.Conn.
 //
+// It's for tunneling arbitrary protocols over WebSockets.
+// Few users of the library will need this but it's tricky to implement
+// correctly and so provided in the library.
+// See https://github.com/nhooyr/websocket/issues/100.
+//
 // Every Write to the net.Conn will correspond to a binary message
 // write on *webscoket.Conn.
 //