add shields for docs, downloads, workflow status

Author: Teque5

README.md

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 <p align="center"><img src="https://github.com/sigmf/SigMF/blob/v1.2.0/logo/sigmf_logo.png" alt="Rendered SigMF Logo"/></p>
 
+![Read the Docs](https://img.shields.io/readthedocs/sigmf)
+![PyPI - Downloads](https://img.shields.io/pypi/dm/sigmf)
+![GitHub Actions Workflow Status](https://img.shields.io/github/actions/workflow/status/sigmf/sigmf-python/main.yml)
+
 This python module makes it easy to interact with Signal Metadata Format
 (SigMF) recordings. This module works with Python 3.7+ and is distributed
 freely under the terms GNU Lesser GPL v3 License.