Adding readme describing the Arduino library.
Adding readme describing the Arduino library.

Adding a document to describe how to build the Arduino library from the
OpenMRN source tree, what platforms and hardware are supported at this
time and to start documenting the common issues observed on the ESP32
platform.

bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn