Document how to use Wraith with logged-in pages

[ChrisBAshton]

We've had a number of issues raised where people need guidance on how to use Wraith to take screenshots of pages which are only available to logged in users, thus requiring a before_capture step prior to taking the screenshots.

It would be helpful to the community if we had a bit of a "how to" guide in the documentation for this scenario, as it seems to be quite a common request.

Relevant issues: #468, #446.

Most detailed answer so far: #446 (comment)