Add lifecycle badge.

README.md

@@ -3,6 +3,7 @@
 [![Code Coverage Status](https://codecov.io/gh/jrnold/ggthemes/branch/master/graph/badge.svg)](https://codecov.io/github/jrnold/ggthemes?branch=master)
 [![rstudio mirror downloads](http://cranlogs.r-pkg.org/badges/ggthemes)](https://github.com/metacran/cranlogs.app)
 [![cran version](http://www.r-pkg.org/badges/version/ggthemes)](http://cran.rstudio.com/web/packages/ggthemes)
+[![lifecycle](https://img.shields.io/badge/lifecycle-stable-brightgreen.svg)](https://www.tidyverse.org/lifecycle/#stable)
 
 
 Some extra geoms, scales, and themes for