Added notice to add username and user_domain_name to the configuration files.

cli/README.md

@@ -30,6 +30,8 @@ it is a form of token-based authentication providing easy and secure access with
    [configure](https://docs.openstack.org/python-openstackclient/rocky/configuration/index.html)
    OpenStack CLI client.
 
+   Remember to specify **username** and **user_domain_name** in **clouds.yaml** or **OS_USERNAME** and **OS_USER_DOMAIN_NAME** in **openrc** file. The values can be found in **Project > API Acess**, click on **View Credentials**. Both values can be found in the **User Name** field (_<username>_@_<user_domain_name>_).
+
    <div style="border-width:0;border-left:5px solid #b8d6f4;background-color:rgba(255,0,0,0.3);margin:20px 0;padding:10px 20px;font-size:15px;">
      <strong>WARNING:</strong><br/><br/>
      Add the following line to the <strong>openrc</strong> file:<br/>