Added UPGRADING doc file that should help to upgrade apps/ports from older versions of lwIP

UPGRADING

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+This file lists major changes between release versions that require
+ports or applications to be changed. Use it to update a port or an
+application written for an older version of lwIP to correctly work
+on a newer version.
+
+
+(CVS HEAD)
+
+  * [Enter new changes just after this line - do not remove this line]
+
+  ++ Application changes:
+
+  * Replaced struct ip_addr by typedef ip_addr_t.
+
+  * Raw API: when calling tcp_abort() from a raw API TCP callback function,
+    make sure you return ERR_ABRT to prevent accessing unallocated memory.
+    (ERR_ABRT now means the applicaiton has called tcp_abort!)
+
+  * Renamed mem_realloc() to mem_trim() to prevent confusion with realloc()
+
+  * Netconn API: Changed netconn_receive() and netconn_accept() to return
+    err_t, not a pointer to new data/netconn.
+
+
+  ++ Port changes
+
+  * Added 3 new files: def.c, timers.c, timers.h
+  
+  * Separated timer implementation from sys.h/.c, moved to timers.h/.c
+
+  * Converted and semaphore functions to take pointers to sys_mbox_t/sys_sem_t;
+
+  * Converted sys_mbox_new/sys_sem_new to take pointers and return err_t;
+
+  * Added Mutex concept in sys_arch (define LWIP_COMPAT_MUTEX to let sys.h use
+    binary semaphores instead of mutexes - as before)
+
+  * Integrated loopif into netif.c - loopif does not have to be created by the
+    port any more, just define LWIP_HAVE_LOOPIF to 1.
+
+  * Netdb uses a memp pool for allocating memory when getaddrinfo() is called,
+    so MEMP_NUM_NETDB has to be set accordingly.
+
+  * Snmp-agent uses memp pools instead of the heap, so MEMP_NUM_SNMP_* have to
+    be set accordingly.
+
+  * Added define LWIP_RAND() for lwip-wide randomization (needs to be defined
+    in cc.h, e.g. used by igmp)
+
+
+(STABLE-1.3.2)
+
+  * initial version of this file