Document that people shouldn't impersonate CCBot

[wumpus]

We recently noticed that someone crawling from idris.fr was impersonating CCBot with the user-agent string.

We contacted IDRIS and they said they would stop doing it.

It would be good for the documentation for img2dataset should say:

1. use your own useragent, don't impersonate anyone else
2. respect robots.txt and X-robots
3. Respect ccbot robots.txt rules and X-robots rules in addition to the rules for your bot