Add README.txt file for Python unit tests.

tools/python/unittests/README.txt

+***************************************
+** Linphone Python module unit tests **
+***************************************
+
+To run these unit tests, you need to have installed the Python module for Linphone
+and to have install the nose unit tests framework.
+Then use this command to run the tests:
+	nosetests -v --nologcapture
+
+The logs from the tests are put in some .log files.
+
+A single test file can be run by specifying it at the command line. For example,
+to run only the message unit tests use:
+	nosetests -v --nologcapture test_message.py