[TYPO3-doc] Screenshot rules and guidelines for Official Documentation

[Philipp Gampe]

> Thanks in advance for your feedback.

One note, please narrow the screen for screenshots as much as necessary 
or go to 1024x768.
The reason for this is, that it is hard so read things in compressed 
screenshots, because they won't fit otherwise.
Reducing the screenshot to the relevant part also helps, but increases 
the risk that someone unfamiliar with TYPO3 loses the orientation (in 
which part he is currently).

Best regards
Phil