Added spi/spi_master_slave. (#101)

Co-authored-by: Michael Stoops <spam@michaelstoops.com>

spi/spi_master_slave/spi_master/CMakeLists.txt

@@ -0,0 +1,12 @@
+add_executable(spi_master
+        spi_master.c
+        )
+
+# Pull in basic dependencies
+target_link_libraries(spi_master pico_stdlib hardware_spi)
+
+# create map/bin/hex file etc.
+pico_add_extra_outputs(spi_master)
+
+# add url via pico_set_program_url
+example_auto_set_url(spi_master)