release notes files in markdown format

bindings/doc/v600/index.md

core/doc/v600/index.md

geom/doc/v600/index.md

graf2d/doc/v600/index.md

graf3d/doc/v600/index.md

gui/doc/v600/index.md

hist/doc/v600/index.md

html/doc/v600/index.md

io/doc/v600/index.md

math/doc/v600/index.md

misc/doc/v600/index.md

montecarlo/doc/v600/index.md

net/doc/v600/index.md

proof/doc/v600/index.md

roofit/doc/v600/index.md

sql/doc/v600/index.md

tmva/doc/v600/index.md

tree/doc/v600/index.md

tutorials/doc/v600/index.md