[m-rev.] trivial diff: update README.MS-VisualC

[Julien Fischer]

Update REAMDE.MS-VisualC:

README.MS-VisualC:
     Mercury has been ported to 64-bit Windows, just not when
     using MSVC as a C compiler.

Julien.

diff --git a/README.MS-VisualC b/README.MS-VisualC
index 217d520..fbda35e 100644
--- a/README.MS-VisualC
+++ b/README.MS-VisualC
@@ -122,7 +122,9 @@ The MSVC port currently has a number of limitation:
    This is due to it containing a number of Unix dependencies.  (Other
    tools that work with deep profiles should be fine.)

-* Mercury has not yet been ported to 64-bit Windows.
+* Generation of 64-bit code using Mercury and MSVC is not yet supported.
+  To generate native 64-bit executables on Windows you will need to use
+  the MinGW64 port (see README.MinGW for details).

  * When used directly from the Windows command prompt, mmake will not work.
    You should use `mmc --make' instead.  (mmake requires a POSIX-like shell