Add URLs to document why DLLIMPORT is needed on Windows.

Per email from Craig Ringer

src/include/port/win32.h

@@ -65,7 +65,13 @@
 
 #define USES_WINSOCK
 
-/* defines for dynamic linking on Win32 platform */
+/* defines for dynamic linking on Win32 platform
+ *
+ *	http://support.microsoft.com/kb/132044
+ *	http://msdn.microsoft.com/en-us/library/8fskxacy(v=vs.80).aspx
+ *	http://msdn.microsoft.com/en-us/library/a90k134d(v=vs.80).aspx
+ */
+ 
 #if defined(WIN32) || defined(__CYGWIN__)
 
 #ifdef BUILDING_DLL