Merge pull request magento-commerce/devdocs#2519 from baranovskyalexandr/Update-general-CLI-installation-guide

Update general cli installation guide

Author: jeff-matthews

src/extensions/install/index.md

@@ -39,6 +39,17 @@ Prior to installation, you may want to:
 To install an extension, you must:
 
 1. Get an extension from the Commerce Marketplace or another extension developer.
+1. If you install an extension from the Commerce Marketplace, make sure that the `repo.magento.com` repository exists in your `composer.json` file:
+
+    ```bash
+   "repositories": [
+       {
+           "type": "composer",
+           "url": "https://repo.magento.com/"
+       }
+   ]
+    ```
+
 1. Get the extension's Composer name and version.
 1. Update the `composer.json` file in your Magento project with the name and version of the extension.
 1. Verify that the extension installed properly.