Update README

Author: mbostock

README.md

@@ -1670,7 +1670,7 @@ The select transform derives a filtered mark index; it does not affect the mark
 
 #### Plot.select(*selector*, *options*)
 
-Selects the points of each series selected by the *selector*, which can be specified either as a function which receives as input the index of the series, or as a {*key*: *value*} object with exactly one *key* being the name of a channel and the *value* being a function which receives as input the index of the series and the channel values. The *value* may alternatively be specified as the shorthand “min” and “max” which respectively select the minimum and maximum points for the specified channel.
+Selects the points of each series selected by the *selector*, which can be specified either as a function which receives as input the index of the series, the shorthand “first” or “last”, or as a {*key*: *value*} object with exactly one *key* being the name of a channel and the *value* being a function which receives as input the index of the series and the channel values. The *value* may alternatively be specified as the shorthand “min” and “max” which respectively select the minimum and maximum points for the specified channel.
 
 For example, to select the point within each series that is the closest to the median of the *y* channel: