Documentation improvement: how to write file

[sanderd17]

When I tried this at first, I always received corrupt files. I opened it with a text editor, and saw a lot more XML than I'd expect in a zip.

The reason was that I used fs.writeFile(fileName, data) instead of fs.writeFile(fileName, data, 'binary').

It might be worth mentioning this in the docs

[pencilcheck]

Perhaps a pull request? Just change the README.