Add cran badge.

Author: jolars

README.md

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 ## RcppParallel Package
 
 [![Travis-CI Build Status](https://travis-ci.org/RcppCore/RcppParallel.svg?branch=master)](https://travis-ci.org/RcppCore/RcppParallel)
+[![CRAN_Status_Badge](http://www.r-pkg.org/badges/version/RcppParallel)](https://cran.r-project.org/package=RcppParallel)
 
 High level functions for doing parallel programming with Rcpp. For example, the `parallelFor` function can be used to convert the work of a standard serial "for" loop into a parallel one and the `parallelReduce` function can be used for accumulating aggregate or other values.