Add git exercises

...ation/Exercices/serie_15_git/1-local_repository/README.md

...tion/Exercices/serie_15_git/2-remote_repository/README.md

...rcices/serie_15_git/2-remote_repository/fig/centralWF.png

...serie_15_git/2-remote_repository/fig/git_flow_command.png

.../serie_15_git/2-remote_repository/fig/gitlab_repo_add.png

...cices/serie_15_git/2-remote_repository/fig/gitlab_ssh.png

...xercices/serie_15_git/2-remote_repository/fig/gl_proj.png

...ices/serie_15_git/2-remote_repository/fig/new_proj_gl.png