docs: add details about pantheon site updates

docs/deployment.md

@@ -18,6 +18,13 @@ Pantheon maintains its own git remote containing the WordPress core files. To de
 1. See these instructions to [add a site to your Pantheon account](https://docs.pantheon.io/guides/getstarted/addsite).
 1. Visit your Pantheon [site dashboard](https://docs.pantheon.io/guides/account-mgmt/workspace-sites-teams/sites#site-dashboard) and make sure your site's Development Mode is set to Git (it will likely be set to SFTP by default).
 ![Pantheon Development Mode](_assets/pantheon-development-mode.png)
+1. Click the "Clone with Git" button, follow the instructions to clone the repo locally.
+1. In the cloned repo, open the `wp-config.php` file.
+1. At the top of the file, directly under the `<?php` line, add this line:
+   - `require __DIR__ . '/vendor/autoload.php';`
+   - This allows the Pantheon site to autoload our vendor dependencies.
+1. Commit the changes to this file.
+1. Push the commit to Pantheon's `master` branch.
 ### Deployment configuration
 
 Deployment to Pantheon requires setting the following variables and secrets in Github. See these instructions for creating [variables](https://docs.github.com/en/actions/learn-github-actions/variables#creating-configuration-variables-for-a-repository) and [secrets](https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions#creating-secrets-for-a-repository).