Shiny metadata in the snap store

[tartley]

Our entry in the snap store looks a bit bare: https://snapcraft.io/universal-ctags

• ~Populate all those fields~ (done)
• ~Consider an icon.~ How about this one: https://iconarchive.com/show/small-n-flat-icons-by-paomedia/tag-icon.html ? (Filed the idea for discussion at https://github.com/universal-ctags/ctags/issues/3298)
• add user documentation, including:
  • ~link to ctags.io and online man pages~ (done)
  • ~names of executables we install~ (done)
  • ~the fact we don't install man pages~ (done)
  • ~cannot access files outside home dir~ (done)
  • ~cannot access hidden home dir files~ (done)
  • ~other known bugs or shortfalls that ctags users ought to know about~ (done)