Add life cycle badge

Author: hadley

README.Rmd

@@ -19,6 +19,7 @@ library(stringr)
 [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/tidyverse/stringr?branch=master&svg=true)](https://ci.appveyor.com/project/tidyverse/stringr)
 [![Coverage Status](https://img.shields.io/codecov/c/github/tidyverse/stringr/master.svg)](https://codecov.io/github/tidyverse/stringr?branch=master)
 [![CRAN Status](http://www.r-pkg.org/badges/version/stringr)](https://cran.r-project.org/package=stringr)
+[![lifecycle](https://img.shields.io/badge/lifecycle-stable-brightgreen.svg)](https://www.tidyverse.org/lifecycle/#stable)
 
 ## Overview
 

README.md

@@ -11,6 +11,7 @@ Status](https://ci.appveyor.com/api/projects/status/github/tidyverse/stringr?bra
 Status](https://img.shields.io/codecov/c/github/tidyverse/stringr/master.svg)](https://codecov.io/github/tidyverse/stringr?branch=master)
 [![CRAN
 Status](http://www.r-pkg.org/badges/version/stringr)](https://cran.r-project.org/package=stringr)
+[![lifecycle](https://img.shields.io/badge/lifecycle-stable-brightgreen.svg)](https://www.tidyverse.org/lifecycle/#stable)
 
 ## Overview