Add an NPM downloads badge to the README

As more badges seem to improve NPM SEO according to NPM's logic.

Signed-off-by: Juan Cruz Viotti <[email protected]>

Author: jviotti

README.markdown

@@ -2,6 +2,7 @@
 
 [![GitHub Release](https://img.shields.io/github/v/release/sourcemeta/jsonschema)](https://github.com/sourcemeta/jsonschema/releases)
 [![NPM Version](https://img.shields.io/npm/v/@sourcemeta/jsonschema)](https://www.npmjs.com/package/@sourcemeta/jsonschema)
+[![NPM Downloads](https://img.shields.io/npm/dm/%40sourcemeta%2Fjsonschema)](https://www.npmjs.com/package/@sourcemeta/jsonschema)
 [![GitHub Actions](https://github.com/sourcemeta/jsonschema/actions/workflows/test.yml/badge.svg)](https://github.com/sourcemeta/jsonschema/actions)
 [![GitHub contributors](https://img.shields.io/github/contributors/sourcemeta/jsonschema.svg)](https://github.com/sourcemeta/jsonschema/graphs/contributors/)
 [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit&logoColor=white)](https://github.com/sourcemeta/jsonschema/blob/main/.pre-commit-hooks.yaml)