Started filling out FSLeyes documentation. Quick start page has stuff on it.

fsleyes_doc/command_line.rst

fsleyes_doc/controls.rst

fsleyes_doc/images/quick_start_interface_1.png

fsleyes_doc/images/quick_start_interface_2.png

fsleyes_doc/images/quick_start_interface_overview.png

fsleyes_doc/images/quick_start_interface_overview.xcf

fsleyes_doc/index.rst

fsleyes_doc/linking_overlays.rst

fsleyes_doc/offscreen_rendering.rst

fsleyes_doc/overlays.rst

fsleyes_doc/overview.rst

fsleyes_doc/perspectives.rst

fsleyes_doc/quick_start.rst