Developer Guide
If you want to modify or extend the simulation environment itself, these pages provide an overview of the system architecture as well as descriptions of the most important components. They are intended to explain the purpose of each component on a higher level than the pure API and give hints about places where one could start modifying the system and possible issues to look out for.
It might also be a good idea to get more familiar with Unity, especially if you want to make changes to the UI or even the visualization of the particle system.