[krbdev.mit.edu #8662] Windows README does not document MFC requirement

[Greg Hudson via RT]

The README file was originally written for MSVC++ 6.0 and was last 
comprehensively updated for Visual Studio 2010 (the version used to 
build the most recent KfW release).  I think perhaps MFC was more 
integral to Visual Studio in earlier releases, but I'd have to do more 
testing to be certain.  We should update the README file regardless.

The build should work without MFC (skipping the Leash build) if you 
"set NO_LEASH=1" in the environment, as we do in the top level of 
appveyor.yml.