Update README.md

The `PYTHON` environment variable should point to python.exe
Update vcbuild.bat

The PYTHON environment variable should be quoted in case there's spaces in the path. For example assume `set PYTHON=C:\some path\python.exe` you'd want the command to be `"C:\some path\python.exe"` so PYTHON would need to be expanded in quotes `"%PYTHON%"`. This is typical in Windows because environment variables usually don't contain quotes and instead you're expected to quote on expansion.
Update vcbuild.bat

amend to last commit. i'm doing this via github's editor in my browser so I don't know if there's a way to do that here but i'm about to find out..