Add 'Requirements' section to README

Author: tomstokes

README.md

@@ -12,6 +12,14 @@ A pure Python SPI interface using the Linux spidev device
 - Supports half-duplex reads and writes as well as full-duplex transfers.
 - Exposes all available spidev interface options as properties.
 
+## Requirements
+
+- spidev enabled in the kernel and (if necessary) the device tree.
+- Write permissions to the /dev/spidevN.N device.
+  - Some distributions have an 'spi' group for this purpose. If available, add this group to the user account and ensure the spidev device is group-writeable.
+  - If no 'spi' group exists, a udev rule can be created to set the permissions of the spidev device.
+  - As a last resort, running the python script as root should allow access to the spidev. **Note** This is not recommended. Use the 'spi' group or udev rules whenever possible.
+
 ## Example
 
     import spi