add diff.sh for diffing manual
add diff.sh for diffing manual

stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math