Update readerscreenshot.lua

This adds screenshot functionality for the Kobo Aura. 

The pam-files generated can be read and converted using Imagemagick for linux/windows.

I've added the infomessage because it takes 5+ tries to get a screenshot by tapping the lower left and upper right corners. The other diagonal is not working for the Kobo Aura yet.
Update readerscreenshot.lua

No the first part of the text can be translated.
Merge pull request #449 from Markismus/patch-3

Update readerscreenshot.lua (Kobo_phoenix)