Add tests/README.md

tests/README.md

+Tests
+=====
+
+To run pyrcb's unit tests, run the file [tests.py](tests.py). The tests should
+pass for both Python 2 and 3.
+
+[mock](https://pypi.python.org/pypi/mock) must be installed to run the tests
+with Python 2 (it is included in Python 3's standard library). No additional
+dependencies are required.
+
+The file [pyrcb.py](pyrcb.py) should be in this directory. It is symlinked
+here, but if the symlink isn't working, you can simply copy the file.