Add service_url config option (#2774)

Doc change for https://github.com/home-assistant/home-assistant/pull/7919

source/_components/volvooncall.markdown

@@ -24,6 +24,23 @@ volvooncall:
   password: password
 ```
 
+Users registered with Volvo in North America or China will need to specify a service url:
+
+```yaml
+# North America 
+volvooncall:
+  username: username
+  password: password
+  service_url: 'https://vocapi-na.wirelesscar.net/customerapi/rest/v3.0/'
+```
+```yaml
+# China
+volvooncall:
+  username: username
+  password: password
+  service_url: 'https://vocapi-cn.wirelesscar.net/customerapi/rest/v3.0/'
+```
+
 A more advanced example for setting the vehicle name and selecting what resources to display:
 
 ```yaml
@@ -43,6 +60,7 @@ Configuration variables:
 
 - **username** (*Required*): The username associated with your Volvo On Call account.
 - **password** (*Required*): The password for your given Volvo On Call account.
+- **service_url** (*Optional*): The service URL to use for Volvo On Call (defaults to https://vocapi.wirelesscar.net/customerapi/rest/v3.0/).
 - **name** (*Optional*): Make it possible to provide a name for the vehicles.
 - **resources** (*Optional*): A list of resources to display (defaults to all available).