docs: add DOI for RDFLib

Add the RDFLib DOI from zenodo to the README using the
"all versions" DOI.

- Closes #1454

README.md

@@ -10,6 +10,7 @@ RDFLib
 [![Downloads](https://pepy.tech/badge/rdflib/week)](https://pepy.tech/project/rdflib)
 [![PyPI](https://img.shields.io/pypi/v/rdflib.svg)](https://pypi.python.org/pypi/rdflib)
 [![PyPI](https://img.shields.io/pypi/pyversions/rdflib.svg)](https://pypi.python.org/pypi/rdflib)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.6845245.svg)](https://doi.org/10.5281/zenodo.6845245)
 
 [![Contribute with Gitpod](https://img.shields.io/badge/Contribute%20with-Gitpod-908a85?logo=gitpod)](https://gitpod.io/#https://github.com/RDFLib/rdflib)
 [![Gitter](https://badges.gitter.im/RDFLib/rdflib.svg)](https://gitter.im/RDFLib/rdflib?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)