Improve README.md

README.md

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-# Tests
+# mesos-journald-container-logger
+
+This is a container logger module for [Mesos](http://mesos.apache.org/)
+that redirects container logs to the systemd journal, or "journald".
+
+Messages written to the container's standard output are logged using the
+priority level `LOG_INFO`, and standard error messages are logged using
+the priority level `LOG_ERR`.
+
+If the environment variable `MESOS_TASK_ID` is defined its value will be
+used as the program identifier in the log messages. This is mostly useful
+if Marathon is used to launch the tasks.
+
+## Tests
 
 You (or the user running the tests) need to to have access to the systemd
-journal. Access is usually granted to members of the "systemd-journal" group.
+journal, or they will fail. Access is usually granted to members of the
+`systemd-journal` group. To add jenkins to this group, for example:
+
+    $ usermod -a -G systemd-journal jenkins
+
+Normal users may need to login again, and you may need to restart services
+such as jenkins, for the group changes to be applied.