index.md : Add large Gaffer logo, links to main page and GitHub, improved calls to action
index.md : Add large Gaffer logo, links to main page and GitHub,
improved calls to action

appleseedhq/gaffer,hradec/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,hradec/gaffer,boberfly/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,boberfly/gaffer,hradec/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,boberfly/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,boberfly/gaffer,johnhaddon/gaffer,hradec/gaffer,andrewkaufman/gaffer,hradec/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer