Update installation instructions through git
Update installation instructions through git

  mention autogen.sh or bootstrap scripts.

laas/hpp-doc,laas/hpp-doc,humanoid-path-planner/hpp-doc,florent-lamiraux/hpp-doc,jmirabel/hpp-doc,jmirabel/hpp-doc,florent-lamiraux/hpp-doc,humanoid-path-planner/hpp-doc