Merge pull request #761 from ARMmbed/AnotherButler-patch-4

Add intro to example in SPI.md

Author: Amanda Butler

docs/api/drivers/SPI.md

@@ -29,4 +29,6 @@ The SPI master generates a clock to synchronously drive a serial bit stream slav
 
 ### SPI hello, world
 
+The following example uses the WHOAMI register. The WHOAMI register is an ID register of the slave device. In other words, it's just an example that you can write to a slave's register. In this example, Mbed OS acts as the SPI master, and DigitalOut acts as the slave chip select (cs).
+
 [![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed_example/code/SPI_HelloWorld/)](https://os.mbed.com/teams/mbed_example/code/SPI_HelloWorld/file/3789302f33bc/main.cpp)