Doc updates (installation, ViscEl, makeClumpCloud)

Few doc updates, for instance

• installation: advertizing possible use of docker for non-Linux OS (if I understood correctly. It is for sure used here for running YADE on a Mac), + note about cmake options consequences about save, after @ppircher suggestion

Plus clarifying

• ViscElPhys.F* model
• the somewhat random initial orientation in makeClumpCloud