persistence.xml is defined by the JPA spec. You can find a sample persistence.xml file here:
but you probably want to tweak it to your needs and database vendor:
https://github.com/Salaboy/jbpm/blob/master/jbpm-persistence-jpa/src/test/resources/META-INF/persistence.xml
Notice that you must place it inside the META-INF directory.
Greetings.