Add arduino-compat readme

PUBLISHED_FROM=4eaff96529f1dc3bbf297de7aad6f306205a454c

Author: dimonomid

README.md

@@ -1 +1,16 @@
-# Arduino compatibility for Mongoose OS
+# Arduino compatibility for Mongoose OS
+
+This library provides a common Arduino compatibility layer, so that one could
+pick an existing Arduino program, throw it into the Mongoose OS application
+sources, and ideally, it "just works".
+
+Currently, the following public headers are provided:
+
+- `Arduino.h`
+- `Print.h`
+- `WString.h`
+- `stdlib_noniso.h`
+
+There are more specific Arduino-compatibility libraries available: for
+[onewire](https://github.com/mongoose-os-libs/arduino-onewire),
+[SPI](https://github.com/mongoose-os-libs/arduino-spi), etc.