Add instructions for running tests.

README.md

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+
+
+# Running The Tests
+
+Your computer should have an Internet connection, and the ``docker`` command
+line tool should be able to connect to a working Docker daemon (localhost or
+elsewhere for OS X etc.) that can run new images.
+
+The IP address of the Docker daemon must allow exposing ports from running
+containers, e.g. it should not be firewalled or port forwarded.
+
+If in doubt, just install Docker on a Linux box in the default configuration
+and run the tests there.