Update copyright PR instructions in README

[ghost]

The README instructions for submitting PRs with new wallpapers says to edit the debian/copyright file, however this file isn't in the master branch (I later found it in the deb-packaging branch). These instructions should be updated to reflect this (and make it easier for new contributors to submit PRs).

@cassidyjames said of this in https://github.com/elementary/wallpapers/pull/99#issuecomment-613658604:

Ah, yeah we need to update the instructions for copyright info (or auto-build that file in CI or something). It's in a different branch, but that requires another PR to update.