Zip that contains directories cannot be imported

Updated the README with the expected zip file structure.

Possible change to support directories in the zip

public function importAssets(...) {
...
if (!is_dir($destination)) {
  $this->fileSystem->move(...)
}

nginex → closed merge request !55

I've provided an extra validation before we import zip, si it's clear to the end user that the structure is not valid.