Display a warning if photos don't have exifs

[TheLastGimbus]

as in #133 discussion:

• [ ] detect how many % of pictures have valid exif with valid date
• [ ] display a waring to user if >15% doesn't
• [ ] ...and link them to guide on how to use exiftool to fix this

[TheLastGimbus]

closing with f3438eeb - there is now always visible exiftool instruction in readme