Update general CLI installation guide

Author: Oleksandr Baranovskyi

src/extensions/install/index.md

@@ -37,8 +37,16 @@ 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 the repo.magento.com repository exists in your composer.json:
+    ```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.