Add test badge to README

Author: mandli

README.txt

@@ -1 +1,7 @@
-This repository will host the classic single-grid Clawpack code.
+# Classic Clawpack
+
+This package hosts the original version of the single-grid Clawpack code.  
+
+![Tests](https://github.com/clawpack/classic/actions/workflows/testing.yml/badge.svg)
+
+- [Documentation](https://www.clawpack.org)