Add a couple of examples of using the Max7219 LED driver chip (#209)

spi/max7219_8x7seg_spi/README.adoc

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+= Attaching a Max7219 driving an 8 digit 7 segment display via SPI
+
+This example code shows how to interface the Raspberry Pi Pico to a Max7219 driving an 8 digit 7 segment display. The particular device used is interfaced via SPI at 3.3v.
+
+The Max7219 datasheet can be found here https://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf
+
+
+== Wiring information
+
+Wiring up the device requires 5 jumpers as follows:
+
+   * GPIO 17 (pin 22) Chip select -> CS on Max7219 board
+   * GPIO 18 (pin 24) SCK/spi0_sclk -> CLK on Max7219 board
+   * GPIO 19 (pin 25) MOSI/spi0_tx -> DIN on Max7219 board
+   * 5v (pin 40) -> VCC on Max7219 board
+   * GND (pin 38)  -> GND on Max7219 board
+
+The example here uses SPI port 0. Power is supplied from the 5V pin.
+
+[NOTE]
+======
+There are many different manufacturers who sell boards with the Max7219. Whilst they all appear slightly different, they all have, at least, the same 5 pins required for power and communication.  Please ensure you connect up as described in the previous paragraph.
+======
+
+== List of Files
+
+CMakeLists.txt:: CMake file to incorporate the example in to the examples build tree.
+max7219_8x7seg_spi.c:: The example code.
+
+== Bill of Materials
+
+.A list of materials required for the example
+[[max7219-bom-table]]
+[cols=3]
+|===
+| *Item* | *Quantity* | Details
+| Breadboard | 1 | generic part
+| Raspberry Pi Pico | 1 | https://www.raspberrypi.com/products/raspberry-pi-pico/
+| max7219 board with 8x7segment LEDs| 1 | generic part
+| M/M Jumper wires | 5 | generic part
+|===