Link to new vignette in README

Author: hadley

README.Rmd

@@ -141,3 +141,5 @@ R provides a solid set of string operations, but because they have grown organic
 * Produces outputs than can easily be used as inputs. This includes ensuring
   that missing inputs result in missing outputs, and zero length inputs
   result in zero length outputs.
+  
+Learn more in `vignette("from-base")`

README.md

@@ -201,3 +201,5 @@ languages like Ruby or Python are rather hard to do in R.
 - Produces outputs than can easily be used as inputs. This includes
   ensuring that missing inputs result in missing outputs, and zero
   length inputs result in zero length outputs.
+
+Learn more in `vignette("from-base")`