GNU Lesser General Public License (LGPL) version 3. See the file COPYING.Lesser.
$ mkdir ~/.FreeCAD/Mod $ cd ~/.FreeCAD/Mod
Install hard dependency for PVC frames.
$ git clone https://github.com/rkrenzler/ose-piping-workbench.git
Install the D3D workbench.
$ git clone https://github.com/skaiser/D3D-Printer-Workbench.git
Create a soft link in your home directory to make it easier to add things like icons from the file browser windows.
$ cd $ ln -s ~/.FreeCAD/Mod/ .
Useful API doc links
These are actually quite helpful:
Also...click Help->Automatic python modules documentation. This is probably what you really want after reading the basic scripting tutorials. However, the thing I've found most helpful is using the interactive python console as introduced in Scripting Basics
The Category:API page has a more browseable list of API docs for a small set of modules than the main API index pages. FreeCAD and FreeCADGui are the main ones to took at, starting out.