Add a badge for a Zenodo DOI.

This allows for easier citations for researchers using the suite.

This DOI will change each time a release is published. How often that
happens is for the moment still ad hoc (i.e. we continually add tests,
and will at some period decide to batch them into a release). In the
future we may decide to automatically publish releases on some cadence.

Author: Julian

README.md

@@ -4,6 +4,7 @@
 [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active)
 [![Financial Contributors on Open Collective](https://opencollective.com/json-schema/all/badge.svg?label=financial+contributors)](https://opencollective.com/json-schema)
 
+[![DOI](https://zenodo.org/badge/5952934.svg)](https://zenodo.org/badge/latestdoi/5952934)
 [![Build Status](https://github.com/json-schema-org/JSON-Schema-Test-Suite/workflows/Test%20Suite%20Sanity%20Checking/badge.svg)](https://github.com/json-schema-org/JSON-Schema-Test-Suite/actions?query=workflow%3A%22Test+Suite+Sanity+Checking%22)
 
 This repository contains a set of JSON objects that implementers of JSON Schema validation libraries can use to test their validators.