Add description to sst help

[rBatt]

I think hadisst is 0.5 deg. Forget years

@JWMorley

[rBatt]

it's 1º on a side

[rBatt]

It goes from 1968 to 2013 right now

[rBatt]

Add following:

#' The data set is formatted as a "RasterBrick" (see \code{\link{raster}}). 
#' It is 1 \degree on a side, and the time horizon is between 1968 and 2013.