Add not about API key expiration

Author: yordan-mitev

_contentTemplates/common/nuget.md

@@ -29,6 +29,8 @@ As the Telerik NuGet server requires authentication, the first step is to obtain
 1. Select **Copy and Close**. Once you close the window, you can no longer copy the generated key. For security reasons, the **API Keys** page displays only a portion of the key.
 1. Store the generated NuGet API key as you will need it in the next steps. Whenever you need to authenticate your system with the Telerik NuGet server, use `api-key` as the username and your generated API key as the password.
 
+>API keys expire after two years. Telerik will send you an email when a key is about to expire, but we recommend that you set your own calendar reminder with information about where you used that key: file paths, project links, AzDO and GitHub Action variable names, and so on.
+
 #end
 
 #reset-store-credentials