tcp: Add reference to initial CWND ietf draft.

Suggested by Alexander Zimmermann
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -196,7 +196,7 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 /* TCP thin-stream limits */
 #define TCP_THIN_LINEAR_RETRIES 6       /* After 6 linear retries, do exp. backoff */
 
-/* TCP initial congestion window */
+/* TCP initial congestion window as per draft-hkchu-tcpm-initcwnd-01 */
 #define TCP_INIT_CWND		10
 
 extern struct inet_timewait_death_row tcp_death_row;