Added a SPI usage example

examples/spi/spi.go

@@ -0,0 +1,49 @@
+/*
+
+SPI example
+
+*/
+
+package main
+
+import (
+	"github.com/stianeikeland/go-rpio"
+	"fmt"
+)
+
+func main() {
+	if err := rpio.Open(); err != nil {
+		panic(err)
+	}
+
+	if err := rpio.SpiBegin(rpio.Spi0); err != nil {
+		panic(err)
+	}
+
+	rpio.SpiChipSelect(0) // Select CE0 slave
+
+	
+	// Send
+	
+	rpio.SpiTransmit(0xFF)             // send single byte
+ 	rpio.SpiTransmit(0xDE, 0xAD, 0xBE) // send several bytes
+
+	data := []byte{'H', 'e', 'l', 'l', 'o', 0}
+ 	rpio.SpiTransmit(data...)          // send slice of bytes
+
+	
+	// Receive
+
+	received := rpio.SpiReceive(5)     // receive 5 bytes, (sends 5 x 0s)
+	fmt.Println(received)
+
+	
+	// Send & Receive
+
+	buffer := []byte{ 0xDE, 0xED, 0xBE, 0xEF }
+	rpio.SpiExchange(buffer)           // buffer is populated with received data
+	fmt.Println(buffer)
+
+	rpio.SpiEnd(rpio.Spi0)
+	rpio.Close()
+}