add guidance on installing type definitions

README.md

@@ -1,7 +1,9 @@
 # CO2.js
 
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
+
 [![All Contributors](https://img.shields.io/badge/all_contributors-13-orange.svg?style=flat-square)](#contributors-)
+
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 <img src="https://github.com/thegreenwebfoundation/co2.js/actions/workflows/unittests.yml/badge.svg" />
@@ -75,6 +77,14 @@ You can also build the CO2.js library from the source code. To do this:
    - `dist/esm` - An ES Modules compatible build.
    - `dist/iife` - An Immediately Invoked Function Expression (IIFE) version of the library.
 
+## Using CO2.js in TypeScript projects
+
+Type definitions for CO2.js are published in the DefinitelyTyped project, and are available on NPM at `@types/tgwf__co2`.
+
+If you want to use type definitions in your project, they should be installed as a devDependency.
+
+`npm install --dev @types/tgwf__co2`
+
 ## Marginal and average emissions intensity data
 
 CO2.js includes yearly average grid intensity data from [Ember](https://ember-climate.org/data/data-explorer/), as well as marginal intensity data from the [UNFCCC](https://unfccc.int/) (United Nations Framework Convention on Climate Change). You can find the data in JSON and CommonJS Module format in the `data/output` folder.