README: add "what software does Glasgow use" section.

revC1: revC? -> revC1

revC1: nudge some traces

README: add "what can I do" section.

README: completely rewrite for clarity and correctness. This is still a rough draft moreso than a complete document.

gateware.fx2_crossbar: fix typo.

revC1: make the serial number field larger still, center actual box

revC1: make serial number box solid white, increase size

revC1: spread apart rear USB testpoints some more

revC1: add testpoints for raw DAC output Also tweak some other testpoint silkscreen markings