Update how Windows ZIP artefacts will be managed

After discussion with @shivammathur and @derickr we are tweaking the requirements around how the Windows binaries will be structured. Key differences:

Windows artefacts attached to GH releases will be a .zip file instead of a .dll
How the contents of the .zip file will be extracted has been defined

ThePHPF / pie-design

Update how Windows ZIP artefacts will be managed #9