Update doxygen/README.md
Update doxygen/README.md

Co-authored-by: Rok Češnovar <8bec8713d4ced84fdc49921cadf08736798069be@fri.uni-lj.si>
stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math