Update windows README for VS2012/Windows SDK 8

The Windows SDK 8 dropped the command-line build environment, so
provide instructions for building with the Visual Studio (2012)
command-line build environment.

https://github.com/krb5/krb5/commit/42095108b86e099a84def9b8f5570818e21e4719
Author: Ben Kaduk <kaduk@mit.edu>
Commit: 42095108b86e099a84def9b8f5570818e21e4719
Branch: master
 src/windows/README |   20 +++++++++++++++-----
 1 files changed, 15 insertions(+), 5 deletions(-)