Add documentation about better alternatives for Specs2

I have been using `junit_xml_listener` for a while with Specs2 (and struggling with a few issues), but I recently discovered that Specs2 has built in support for JUnit XML output which works much better than `junit_xml_listener`.

See http://stackoverflow.com/questions/6934868/configuring-junitxml-output-for-specs2-tests-in-sbt-0-10
and http://stackoverflow.com/questions/19033473/how-to-integrate-the-junitxml-listener-to-specs2

To save others the same trouble, I think this could be documented in the project readme (at least until the `junit_xml_listener` support for Specs2 catches up with the built in formatter).