Merge branch 'doc/groups' into 'master'

doc/workflow/README.md

doc/workflow/groups.md

doc/workflow/groups/add_member_to_group.png

doc/workflow/groups/group_dashboard.png

doc/workflow/groups/group_with_two_projects.png

doc/workflow/groups/new_group_button.png

doc/workflow/groups/new_group_form.png

doc/workflow/groups/override_access_level.png

doc/workflow/groups/project_members_via_group.png

doc/workflow/groups/transfer_project.png