Hints for ristretto in main

Hints

• asv-description-first-para-too-short
  :73 - Ristretto is an image viewer for the Xfce desktop environment.
  The first `description/p` paragraph of this component might be too short (< 80 characters). Please consider starting with a longer paragraph to improve how the description looks like in software centers and to provide more detailed information on this component immediately in the first paragraph.