Add developer docs section for PHPUnit test execution via wp-env

docs/developer/README.md

@@ -88,6 +88,15 @@ In fact, after the initial setup, you may notice that in your code editor, there
 
 ![Development Site Plugins/Themes](../media/gitignore—gatherpress.png)
 
+#### Running PHPUnit Tests (`wp-env`)
+
+1. Install Node. [NVM](https://github.com/nvm-sh/nvm) is recommended, using `nvm use` in the GatherPress directory before any `npm` command.
+2. Install [Composer](https://getcomposer.org/doc/00-intro.md)
+3. In your terminal window, `cd` to the `gatherpress` directory
+4. Run `npm install` to get node depdendencies
+5. Run `composer install` to get PHPUnit dependencies
+6. Run `npm run test:unit:php` to execute tests
+
 #### To shut down your development session
 
 Simply run: