DOC: update doctr instruction

[ke-zhang-rd]

Reason: https://github.com/drdoctr/doctr/blob/0f19ff78c8239efcc98d417f36b0a31d9be01ba5/doctr/travis.py#L593

It's very hard to find in Travis log. Think need to mention in instruction.

[asmeurer]

FWIW, forked repos will be supported once I finish drdoctr/doctr#343, assuming that you configure doctr for that repo. The main issue is if you configure a base repo then a fork cannot work, as Travis doesn't give forks or forked pull requests access to the secure environment variables for obvious reasons.

[mrakitin]

I think this warning can be better represented via a warning block.

I also think the link from @asmeurer's #79 (comment) can be mentioned as well.

[mrakitin]

Looks good. Here are a couple of my suggestions.

[mrakitin]

A note to ourselves: it's not necessary to have a blank line between .. warning:: and the text, it renders fine by sphinx:

[danielballan]

Good to know. At least some directives will be confused by the lack of a blank line, as they will interpret the line beneath .. directive:: as arguments/parameters so as a matter of style/linting the blank line is probably a good habit just to avoid tripping over that when it does come up.

[mrakitin]

Sounds good, will do in the future.