Documentation

[olivier-stasse]

From recent comments and issues open previously on github, it appears clearly that a documentation is needed, at least at the user level. The recent merge with the fusion merge proposed a very different way of specifying a robot model. The integration of the appendix A of R. Featherstone is also calling for some explanation for possible extensions,
aka being able to handle joints with various representations (for instance Free flyer with rpy, matrix, or quaternion representations with various orders X-Y-Z, Z-Y-X Euler angles).