README gains archive note (#23)

Relates to #22

Author: jdhoffa

README.md

@@ -1,5 +1,9 @@
 # pacta.r.package <img src="man/figures/logo.png" align="right" width="120" />
 
+[![Project Status: Unsupported](https://www.repostatus.org/badges/latest/unsupported.svg)](https://www.repostatus.org/#unsupported)
+
+**This project is archived for future reference, but no new work is expected in this repository.**
+
 <!-- badges: start -->
 [![Lifecycle: experimental](https://img.shields.io/badge/lifecycle-experimental-orange.svg)](https://lifecycle.r-lib.org/articles/stages.html#experimental)
 [![Codecov test coverage](https://codecov.io/gh/RMI-PACTA/pacta.r.package/branch/main/graph/badge.svg)](https://app.codecov.io/gh/RMI-PACTA/pacta.r.package?branch=main)