#167 Add URL to README budge (Downloads)

Author: astropenguin

README.md

@@ -2,7 +2,7 @@
 
 [![Release](https://img.shields.io/pypi/v/xarray-dataclasses?label=Release&color=cornflowerblue&style=flat-square)](https://pypi.org/project/xarray-dataclasses/)
 [![Python](https://img.shields.io/pypi/pyversions/xarray-dataclasses?label=Python&color=cornflowerblue&style=flat-square)](https://pypi.org/project/xarray-dataclasses/)
-![Downloads](https://img.shields.io/pypi/dm/xarray-dataclasses?label=Downloads&color=cornflowerblue&style=flat-square)
+[![Downloads](https://img.shields.io/pypi/dm/xarray-dataclasses?label=Downloads&color=cornflowerblue&style=flat-square)](https://pepy.tech/project/xarray-dataclasses)
 [![DOI](https://img.shields.io/badge/DOI-10.5281/zenodo.4624819-cornflowerblue?style=flat-square)](https://doi.org/10.5281/zenodo.4624819)
 [![Tests](https://img.shields.io/github/workflow/status/astropenguin/xarray-dataclasses/Tests?label=Tests&style=flat-square)](https://github.com/astropenguin/xarray-dataclasses/actions)