Warn that members won't be invited to the new room in room settings

res/css/views/settings/tabs/_SettingsTab.scss

@@ -14,6 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+.mx_SettingsTab_warningText {
+    color: $warning-color;
+}
+
 .mx_SettingsTab_heading {
     font-size: 20px;
     font-weight: 600;