This file was originally written by Richard Hughes, for Fedora, who
proposed it on the Transmission forum here:
https://forum.transmissionbt.com/viewtopic.php?f=3&t=16443
It's used to display a nice description and screenshot in GNOME
Software. The format is documented here:
https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
The <_name/> and <_summary/> fields match the Name= and Comment= fields
in the .desktop file so are already translated.
The screenshot is taken with Transmission 2.92 (14714) on Fedora 25.
It's at one of the sizes recommended by the GNOME Shell Screenshot
Window Sizer extension.
The <update_contact/> is based on the Git history.
Will Thompson