bpo-31691: Specify where to find build instructions for the Windows installer (#4426)

PCbuild/readme.txt

@@ -25,6 +25,8 @@ external dependencies. To build, simply run the "build.bat" script without
 any arguments. After this succeeds, you can open the "pcbuild.sln"
 solution in Visual Studio to continue development.
 
+To build an installer package, refer to the README in the Tools/msi folder.
+
 The solution currently supports two platforms.  The Win32 platform is
 used to build standard x86-compatible 32-bit binaries, output into the
 win32 sub-directory.  The x64 platform is used for building 64-bit AMD64