Add SPI usage to README.

Author: tylercrumpton

README.md

@@ -6,6 +6,7 @@ Introduction
 
 `pyduino-includes` is a SNAPpy library that is designed to make development for the Synapse Pyduino board easier. It
 allows references to the nifty Pyduino IO names (like D3, AD0, or SCL) instead of the underlying SNAPpy GPIO numbers.
+It also provides a bit-banged SPI implementation that can be used for shields that utilize the SPI pins.
 
 Installation
 ------------
@@ -42,3 +43,13 @@ Pins can be referenced as follows:
 | Analog   | A0 - A5         |
 | i2c      | SDA, SCL        |
 
+Setting up the SPI pins is very simple, too:
+
+```python
+from PyduinoIncludes.SPI import *
+
+def my_spi_function():
+    spi_init()  # Sets up the bit-banged SPI
+    spi_write("\x12\x34\x56")  # Write data to the SPI bus
+    spi_read(4)  # Read four bytes from the SPI bus
+```