Add readme with example LwIP address initialization.

ports/lwip/README.md

@@ -0,0 +1,46 @@
+## Port of BACnet/IP to LwIP 2.x
+
+This LwIP BACnet/IP port uses a Makefile designed to be used from
+the continuous integration pipeline. The container is configured with 
+Ubuntu Linux and uses APT to install liblwip-dev library and header files.
+This build method ensures that the code is valid for the particular LwIP 
+library that is released with Ubuntu.
+
+## Integration Hints
+
+Developer must set the IP address, netmask, and UDP port into the BACnet/IP module.
+
+Integration used a main loop bare metal design, shown here as an example.
+
+    int main(void)
+    {
+        static bool valid_ip_address = false;
+        static bool bacnet_ip_initialized = false;
+
+        bacnet_init();
+        LwIP_Init();
+        /* Infinite loop */
+        for (;;) {
+            /* Periodic tasks */
+            LwIP_Periodic_Handler();
+            /* Run when there is a valid IP address */
+            if (netif.ip_addr.addr != 0) {
+                if (!valid_ip_address) {
+                    valid_ip_address = true;
+                    bip_set_addr(netif.ip_addr.addr);
+                    bip_set_broadcast_addr(
+                        (netif.ip_addr.addr & netif.netmask.addr) |
+                        (~netif.netmask.addr));
+                } else {
+                    if (bacnet_ip_initialized) {
+                        bacnet_task();
+                    } else {
+                        bip_init(NULL);
+                        bacnet_ip_initialized = true;
+                    }
+                }
+            } else {
+                valid_ip_address = false;
+            }
+        }
+    }