readme: add demo of terminal emulators in a 3D environment

[marceline-cramer]

The current terminal emulator demo screenshot is cool but not the best hook for Hearth, since while it represents the "self-modifying" part of Hearth, it doesn't include the "virtual world" part of Hearth. We should update it with a screenshot of a full 3D virtual environment featuring terminal emulators in some key role.