Update spi/README.md

SDA doesn't figure in the text, only the datasheet.

According to http://elinux.org/RPi_BCM2835_GPIOs, SPI0 has only 2 chip selects.
The datasheet mentions a Chip Select 2 for the CS register, but this appears to be an error.
Merge pull request #99 from notro/master

Update spi/README.md