Add note about having to add some files to the VC 4.2 files.

PC/readme.txt

@@ -70,6 +70,11 @@ Microsoft Visual C++ Version 4.x (32-bit Windows)
 (For historic reasons this uses the filename "vc40"; it has been tested
 most recently with VC 4.2.  See below for VC 5.x.)
 
+NOTE: VC 4.2 support is eroding, as I no longer have a VC 4.2
+installation.  Some newer files need to be added to the project, in
+particular, reopmodule.c, pcremodule.c, and pypcre.c (all from the
+Modules subdirectory); this list may grow.
+
 The distributed Makefile is vc40.mak.  This file is distributed with
 CRLF line separators, otherwise Developer Studio won't like it.  It
 will NOT work from this PC directory.  To use it, first copy it to the