Add lifecycle badge to README (#1903)

Inspired by #1885 (comment).

Maybe it would be good to clarify that the API is mature and stable upfront, without having to check version numbers in DESCRIPTION, assuming the reader is even familiar with SemVer.

README.md

@@ -3,6 +3,7 @@
 [![R build status](https://github.com/r-lib/lintr/workflows/R-CMD-check/badge.svg)](https://github.com/r-lib/lintr/actions)
 [![codecov.io](https://codecov.io/github/r-lib/lintr/coverage.svg?branch=main)](https://codecov.io/github/r-lib/lintr?branch=main)
 [![CRAN_Status_Badge](https://www.r-pkg.org/badges/version/lintr)](https://cran.r-project.org/package=lintr)
+[![lifecycle](https://img.shields.io/badge/lifecycle-stable-brightgreen.svg)](https://lifecycle.r-lib.org/articles/stages.html)
 
 `{lintr}` provides [static code analysis for R](https://en.wikipedia.org/wiki/Static_program_analysis). It checks for adherence to a given style, identifying syntax errors and possible semantic issues, then reports them to you so you can take action. Watch lintr in action in the following animation: