Add readme file

test/functional/README.md

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+## Functional test suite
+
+This directory contains the functional test suite for the project. This relies on the IoT Agent Node Lib Functional test
+suite. For further information, visit the
+[documentation](https://github.com/telefonicaid/iotagent-node-lib/tree/master/test/functional).
+
+The `functional-tests-runner.js` script is used to run the functional test suite. It is similar to the one used in the
+IoT Agent Node Lib, but it has been adapted to the particularities of the IoT Agent JSON. This script imports both the
+IoT Agent Node Lib `testCases.js` and the local `testCases.js` file. The latter contains the test cases that are
+specific to the IoT Agent JSON, while the former contains the test cases that are common to all IoT Agents.
+
+If you plan to include a tests for an specific feature of the IoT Agent JSON, please, consider added it into the IoTA
+Node Lib `testCases.js` file and use the skip feature to avoid running it for other agents that do not support it. You
+can check the documentation of the IoT Agent Node Lib Functional test suite linked previously for further information.
+
+Additionally, the `functional-tests.js` file is a simple example of how to implement code bases tests using the IoT
+Agent Node Lib Functional test suite utilities. (This test is coded implemented and suites more complex cases than the
+ones contained in the `testCases.js` file).