Add nightly-build status badge to README (#311)

Author: brian-rose

README.md

@@ -1,6 +1,6 @@
 # Project Pythia Portal
 
-![GitHub Workflow Status (branch)](https://img.shields.io/github/workflow/status/NCAR/pythia-portal/deploy-site/main?logo=github&style=for-the-badge)
+[![nightly-build](https://github.com/ProjectPythia/projectpythia.github.io/actions/workflows/nightly-build.yaml/badge.svg)](https://github.com/ProjectPythia/projectpythia.github.io/actions/workflows/nightly-build.yaml)
 
 This is the source repository for the [Project Pythia portal](https://projectpythia.org).
 The portal site is built with [sphinx](https://www.sphinx-doc.org/).