Documentation for tools, modding and tutorials can be found at http://wiki.frictionalgames.com
You should check the HPL3 section and if you want to contribute you can do so in the HPL3/Community/ section.

Getting started video tutorials:
https://wiki.frictionalgames.com/hpl3/tutorials/videos/hpl3_video_tutorials

On the frictional games forum there is also a section for discussing and helping each other with mod development at https://www.frictionalgames.com/forum/forum-79.html

A good starting point should be this community created getting started article (work in progress) https://wiki.frictionalgames.com/hpl3/community/hpl3_getting_started