I’m planning to improve the plugin docs in the next couple days, including logging plugins.
Basically you need to create a new class that extends liquibase.logging.core.AbstractLogger and overrides the getPriority method to return larger than 1 and the debug(), info() etc. methods. If you make the class in a sub-package of liquibase.ext it will automaticaly be picked up by liquibase and used.
I was in the process of moving the java.util.logging logger to the extension portal (http://liquibase.org/extensions) but haven’t gotten it up yet. Hopefully this weekend.
Nathan