Add readme for arduino-spi

PUBLISHED_FROM=87c1036709911d7ab40c7c1d33a9e97c4d283c85

Author: dimonomid

README.md

@@ -1 +1,6 @@
 # Arduino SPI library for Mongoose OS
+
+This library provides an Arduino compatibility layer for SPI by providing
+an `SPI.h` public header, so that one could pick an existing Arduino
+program which uses SPI, throw it into the Mongoose OS application sources,
+and ideally, it "just works".