Add SPI-page
Make a guide for interfacing SPI-based sensors. Topics that needs to be added:
- Clock polarity is active low
- Phase is leading edge
- Shifts out most significant bit first
- SentiBoard operates on 16 bits at the time outputting the most significant byte of those 16 bits first. VERY impportant when you configure the poll string
- The importance of saving and restarting the board immediately after changing the poll string