Add link to testing guide to the development docs

Author: khanniie

developer_docs/README.md

@@ -4,6 +4,7 @@ This folder contains documents intended for developers of the p5.js Web Editor.
 * [Contribution Guide](https://github.com/processing/p5.js-web-editor/blob/develop/.github/CONTRIBUTING.md) - A place to get started!
 * [Installation](installation.md) - A guide for setting up your development environment
 * [Development](development.md) - A guide for adding code to the web editor
+* [Testing](./testing.md) - A guide for writing and running tests in the codebase
 * [Preparing a pull-request](preparing_a_pull_request.md) - Instructions for how to make a pull-request
 * [Accessibility Guidelines](accessibility.md) - Guidelines for writing code to create an accessible application
 * [Translations Guidelines](translations.md) - Guidelines for translating the application