Add required label "alertname" to POST-example (#1254)

We figured out that the label 'alertname' is required to get a proper response from AlertManager.
Signed-off-by: Decker, Stefan <Stefan.Decker@gdata.de>

content/docs/alerting/clients.md

@@ -21,6 +21,7 @@ the following format:
 [
   {
     "labels": {
+      "alertname": "<requiredAlertName>",
       "<labelname>": "<labelvalue>",
       ...
     },