Update Readme for Screenshot Generator

[nicklamonov]

[A task for a PMM]

For Actor https://apify.com/apify/screenshot-url the README is really outdated.

Since the Actor has 300+ monthly users, it needs to be updated and improved.

[nicklamonov]

Issue within the original repo: apify-projects/store-screenshot-url#23
It's not possible to include this in the current board, so we would keep this issue open too for tracking purposes until the original is resolved.