Document Button Name(s) and Function(s) & Connectors On Main README.md

[ForrestErickson]

Add photos which show the buttons and document the function.

There appear to be three buttons to the left of the display.

[ForrestErickson]

The image of the T4.0 VenMon appears to have two buttons. That image should also be annotated.
A Summary Description of what the buttons do should be in the main README.md.

[ForrestErickson]

Name and label these connectors