Added unit test badge into readme

Author: geoffroy-noel-ddh

README.md

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+![Unit Tests](https://github.com/kingsdigitallab/kdl-dts-server/actions/workflows/npm-test.yml/badge.svg?branch=main)
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+
 Server-side javascript implementation of a [Distributed Text Services (DTS)](https://distributed-text-services.github.io/specifications/) server fed from a corpus of [TEI](https://tei-c.org/) files.
 
 Status: this package is currently customised for the [Alice Thornton's Books research project](https://github.com/kingsdigitallab/alice-thornton). The intention is to generalise it so it works with other projects.