Add work around description in README

Author: mike919192

source/portable/NetworkInterface/Zynq/README.txt

@@ -40,3 +40,11 @@ It is obligatory to define:
 
 #define ipconfigZERO_COPY_RX_DRIVER               1
 #define ipconfigZERO_COPY_TX_DRIVER               1
+
+If using SDT drivers, it may be necessary to define certain link speed divisor values.
+This is to work around a driver issue where certain link speeds will not transmit any data
+without defining the values.
+For example, it may be necessary to define:
+
+#define XPAR_PS7_ETHERNET_0_ENET_SLCR_100MBPS_DIV0 8
+#define XPAR_PS7_ETHERNET_0_ENET_SLCR_100MBPS_DIV1 5