Document the default screenshot directory

[DanKaplanSES]

My test calls generateImage(); without issue, but I have no idea where the screenshot went. This is probably documented within the screenshot options, but the link to that in the readme 404s. Regardless, it might be worth stating directly in the readme of this project as I'm sure it's something any first-time user would want to know.

[DanKaplanSES]

I just noticed this function returns a Promise but the first example doesn't await on it. That's why it gave me no information when I called generateImage(); and my test seemed to pass. I don't have a lot of experience writing top-level async/await code; does this first example work correctly or is it also missing an await like my test was?