README: Switch symbol pins to 50 mil grid

Schematic capture by default uses a 50 mil grid, so keeping symbol pin
ends on that grid should work well and provides more flexibility when
creating symbols.

But keep pins spaced 100 mils from each other so that having pin texts
which indicate the pin number sized at 50 mils does not look silly.

This required a renumbering of many of the schematic symbol rules.

Signed-off-by: Andrew Bradford <andrew@bradfordembedded.com>