Update README.md (#28)

README.md

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 # pydrobert-param
 
 [![Documentation Status](https://readthedocs.org/projects/pydrobert-param/badge/?version=latest)](https://pydrobert-param.readthedocs.io/en/latest/?badge=latest)
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+[![Build status](https://github.com/sdrobert/pydrobert-param/actions/workflows/tox.yml/badge.svg)](https://github.com/sdrobert/pydrobert-param/actions/workflows/tox.yml)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 
 Utilities for the python package [param](http://param.pyviz.org/).