Add SPI HelloWorld example (#104)

* Add SPI HelloWorld example

* Edit README.md

Edit file for missing articles and formatting.

Co-authored-by: Amanda Butler <[email protected]>

Author: mprse

APIs_Drivers/SPI_HelloWorld/README.md

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+# SPI example
+
+This example shows how to use the SPI interface to read the WHOAMI register of the slave device. The WHOAMI register is an ID register of the slave device.
+
+**Note:** Please update your MISO, MOSI, SCLK and CS pins.

APIs_Drivers/SPI_HelloWorld/main.cpp

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+/*
+ * Copyright (c) 2006-2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "mbed.h"
+
+SPI spi(D11, D12, D13); // mosi, miso, sclk
+DigitalOut cs(D0);
+
+int main()
+{
+    // Chip must be deselected
+    cs = 1;
+
+    // Setup the spi for 8 bit data, high steady state clock,
+    // second edge capture, with a 1MHz clock rate
+    spi.format(8, 3);
+    spi.frequency(1000000);
+
+    // Select the device by seting chip select low
+    cs = 0;
+
+    // Send 0x8f, the command to read the WHOAMI register
+    spi.write(0x8F);
+
+    // Send a dummy byte to receive the contents of the WHOAMI register
+    int whoami = spi.write(0x00);
+    printf("WHOAMI register = 0x%X\n", whoami);
+
+    // Deselect the device
+    cs = 1;
+}