warn against mixing rpio SPI with spi-bcm2835 + spidev

[mharsch]

rpio SPI can't coexist with the regular linux SPI kernel module (at least for a given spi bus). Running a rpio SPI script on a system with spi-bcm2835 loaded will corrupt the state of the kernel module and break any subsequent calls to it's interface (silently). We should include a warning in the documentation.

[jperkin]

Yeh this is the same as regular GPIO if you try to mess via /sys/class at the same time. I'll add a note.

[mightydread]

Hi... can you explain a bit more? I'm trying to write an script with this library and have issues with the spi on a Rpi 3 with the spi-bcm2835. but if I blacklist it no spi devices appear under /dev