document ridges #74

ridges diverges from R implementation. its not a geom.
when you call ggridges(col="abc", overlap=1.3)
it uses the underlying grid system to create views.
without overlap 5 views will be 20% height each. with overlap the height is more and they endup overlapping.
after that ggplot is used as usual.
if you do geom_point geom_density geom_area etc, every view and its segmented data will draw that.
because this differs from the default implementation we need to have good documentation about it.