Add badges

Author: groenroos

README.md

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 # Nunjucks driver for Sapling
 
+<a href="https://github.com/saplingjs/render-driver-nunjucks/actions"><img src="https://img.shields.io/github/workflow/status/saplingjs/render-driver-nunjucks/CI" alt="GitHub Workflow Status"></a>
+<a href="https://app.codecov.io/gh/saplingjs/render-driver-nunjucks"><img src="https://img.shields.io/codecov/c/gh/saplingjs/render-driver-nunjucks?token=E3NRW4IJW9" alt="Codecov"></a>
+<a href="https://www.npmjs.com/package/@sapling/render-driver-nunjucks"><img src="https://img.shields.io/npm/v/@sapling/render-driver-nunjucks.svg?sanitize=true" alt="Version"></a>
+<a href="https://github.com/saplingjs/render-driver-nunjucks/blob/master/LICENSE"><img src="https://img.shields.io/npm/l/@sapling/render-driver-nunjucks.svg?sanitize=true" alt="License"></a>
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 This package allows using the [Nunjucks](https://mozilla.github.io/nunjucks/) templating engine with [Sapling](https://github.com/saplingjs/sapling/).