After Chapter 2. Getting Started, Section Demo setup, add a section "Library dependencies" that explains the general principles of the library dependencies:
The PVM module runs without external library dependencies
Typical activiti-engine usage requires these libs
When using the activiti-engine with Spring also these libs are required
Many dependencies are introduced for specific features. E.g. JavaMail, JMS, etc. Those libs are only required when the features are used. But we can't really manage documenting or automating the dependencies at that fine level. So we only define the full list of dependencies for all the BPMN and Activiti Engine features.