Add better description and links to README

Author: mandli

README.md

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+# Classic Clawpack
+
+This package hosts the single-grid version of the Clawpack code.  All of the Clawpack solvers include the *classic* algorithms described in [LeVeque-FVMHP](https://www.clawpack.org/fvmhp_materials/) [DOI](https://doi.org/10.1017/CBO9780511791253).
+
+**Links**
+ - [Documentation](https://www.clawpack.org/)
+ - ![Tests](https://github.com/clawpack/classic/actions/workflows/testing.yml/badge.svg)