fixed isFileExist to use full path
updated readme with note to clone the repo and not download the zip version
Merge pull request #3858 from MSOpenTech/winrt-readme-fix

[ci skip]Winrt readme fix
Merge pull request #3851 from MSOpenTech/winrt-fileutils-fix

[ci skip]Winrt fileutils fix
Merge remote-tracking branch 'upstream/master'