Add simplified wiring diagram to README

[dihm]

As reminder in #81, we need to put a wiring diagram in the README so people aren't required to open the KiCAD project just to figure out what pin goes to where.

[dihm]

#84 Includes the diagram, but I just now noticed it has an error. The Clock Input for the DDS Sweeper should actually be a clock output going from the sweeper to the AD9959.

Need to make another PR to fix that now that I merged it without looking it over again.

[dihm]

Fixed by #84 and #85