Refine README.md

Add screenshots; emphasise that this is server-side scripts for
developers/sysadmins not general consumers; other minor tweaks.

Author: penyuan

.reuse/dep5

@@ -10,6 +10,8 @@ Files:
     docs/images/dashboard_flowchart.drawio.jpg
     docs/images/dashboard_flowchart.drawio.svg
     docs/images/hover-plot-controls.gif
+    docs/images/osdash-screenshot.jpg
+    docs/images/osmine-example-run.jpg
 Copyright: 2021 Pen-Yuan Hsing
 License: CC-BY-SA-4.0