added README

Author: cmaglie

extras/test/README.md

@@ -0,0 +1,30 @@
+## Unit test for the Arduino_RPClite library
+
+This directory contains unit-tests that can run locally on PC.
+The transport is mocked, no external hardware is required to run the tests.
+
+### Running the test
+
+Prerequisites:
+
+* `cmake`
+* A working C++ compiler.
+
+From a terminal, change directory to this folder, and run:
+
+```
+mkdir build
+cd build
+cmake ..
+make
+```
+
+it should compile the test and output the executable binary in `build/bin/test-Arduino_RPClite`.
+
+The output should look similar to the following:
+
+```
+$ bin/test-Arduino_RPClite
+===============================================================================
+All tests passed (91 assertions in 1 test case)
+```