git: openjdk/shenandoah-visualizer: master: Add screenshot to README

[duke]

Changeset: 72961647
Author:    Aleksey Shipilëv <shade at redhat.com>
Committer: GitHub <noreply at github.com>
Date:      2020-11-16 20:20:21 +0000
URL:       https://github.com/openjdk/shenandoah-visualizer/commit/72961647

Add screenshot to README

! README.md