Updated CONTRIBUTING.md to reflect new build process

.github/CONTRIBUTING.md

@@ -42,16 +42,30 @@ https://woocommerce.com/contact-us/
 - [WooExperts](https://woocommerce.com/experts/)
 - [Codeable](https://codeable.io/)
 
+## Build process
+
+Compiled CSS and JS files are not included in WooCommerce, also some projects like Blocks and REST API, so it's necessary to build ALL dependencies and assets when downloading or cloning WooCommerce source code from GitHub.
+
+In a terminal go to your WooCommerce installation, and run:
+
+```bash
+npm install
+composer install
+npm run build
+```
+
+Note that it's required to have installed [Node.js to run NPM](https://nodejs.org/en/) and [Composer](https://getcomposer.org/).
+
 ## Coding Guidelines
 
 - **Ensure you stick to the [WordPress Coding Standards](https://make.wordpress.org/core/handbook/best-practices/coding-standards/php/)**
-- Install our pre-commit hook using composer. It'll help with the Coding Standards. To install run `composer install` from the command line within the woocommerce plugin directory.
+- Run our build process described in the section above, it will install our pre-commit hook, code sniffs, dependencies, and more.
 - Ensure you use LF line endings in your code editor. Use [EditorConfig](http://editorconfig.org/) if your editor supports it so that indentation, line endings and other settings are auto configured.
 - When committing, reference your issue number (#1234) and include a note about the fix.
 - Ensure that your code is compatible with PHP 5.2+.
 - Push the changes to your fork and submit a pull request on the master branch of the WooCommerce repository. Existing maintenance branches will be maintained by WooCommerce developers.
 
-Please **don't** modify the changelog or update the .pot files. These will be maintained by the WooCommerce team.
+Please **don't** modify the changelog, it will be maintained by the WooCommerce team.
 
 ## Translating WooCommerce