added skew normal to to-do along with more notes on cdfs and log cdfs and ccdfs
added skew normal to to-do along with more notes on cdfs and log cdfs and ccdfs

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