restore CRAN link [ci skip]

Author: eddelbuettel

README.md

@@ -66,8 +66,9 @@ available for code relying on the older interface. New development should
 always use this Rcpp package instead.
 
 Other usage examples are provided by packages using Rcpp. As of January 2016, there
-are about 550 CRAN packages using Rcpp, a further [BioConductor](http://www.bioconductor.org) packages as
-well as an unknown number of GitHub, Bitbucket, R-Forge, ... repositories using
+are about 550 [CRAN](http://cran.r-project.org) packages using Rcpp, a further
+[BioConductor](http://www.bioconductor.org) packages as well as an unknown
+number of GitHub, Bitbucket, R-Forge, ... repositories using 
 Rcpp.  All these packages provide usage examples for Rcpp.