Add logo.png

Hi and thank you for developing plasma-phonebook!

As far as I know, Gitlab automatically recognises project's icon when a file logo.png is placed in the repo's root path. My experience in cloning this repo to prepare package for Debian has been, logo is actaully not recognised.

As I've just opened the SVG in Gimp and exported the PNG, image copyright stays at the copyright holder for original org.kde.phonebook.svg file.