Add npm badge.

Author: davidlehn

README.md

@@ -3,6 +3,7 @@ jsonld.js
 
 [![Build status](https://img.shields.io/github/workflow/status/digitalbazaar/jsonld.js/Node.js%20CI)](https://github.com/digitalbazaar/jsonld.js/actions?query=workflow%3A%22Node.js+CI%22)
 [![Coverage status](https://img.shields.io/codecov/c/github/digitalbazaar/jsonld.js)](https://codecov.io/gh/digitalbazaar/jsonld.js)
+[![npm](https://img.shields.io/npm/v/jsonld)](https://npm.im/jsonld)
 
 Introduction
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