docs: Better document writing binary data

[OS-pedrogustavobilro]

To address #40

@alextreppass / @kzKallisti / @HarelM feel free to leave feedback!

[HarelM]

This looks good, thanks! I would consider addind a base64 encoding option to explicitly say "hey I'm passing a base64 string", right now you have to pass "undefined" (or omit encoding) if you pass a base64 string. You might consider also adding something like:
If the provided content is not a base64 string this will throw an error, although this might be implied anyway.