add badge

Author: kevinushey

README.md

@@ -1,9 +1,15 @@
 RcppRoll
-=====
+========
+
+[![CRAN status](https://www.r-pkg.org/badges/version/RcppRoll)](https://cran.r-project.org/package=RcppRoll)
 
 This package provides windowed-versions of commonly-used mathematical
 and statistical functions.
 
-Install me with devtools:
+Install the latest release from CRAN with:
+
+    install.packages("RcppRoll")
+
+Or, install the development version with:
 
     install_github("kevinushey/RcppRoll")