docs: Adding info about template for Grafana service (#386)

* Adding info about template

Signed-off-by: lrochette <[email protected]>

* signing off

Signed-off-by: lrochette <[email protected]>

---------

Signed-off-by: lrochette <[email protected]>

Author: lrochette

docs/services/grafana.md

@@ -37,7 +37,22 @@ stringData:
   grafana-api-key: api-key
 ```
 
-7. Create subscription for your Grafana integration
+7. Create a template in `argo-notifications-cm` Configmap
+This will be used to pass the (required) text of the annocation to Grafana (or re-use an existing one)
+As there is no specific template for Grafana, you must use the generic `message`:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: argocd-notifications-cm
+data:
+  templates:
+    template.app-deployed: |
+      messsage: Application {{.app.metadata.name}} is now running new version of deployments manifests.
+```
+
+8. Create subscription for your Grafana integration
 
 ```yaml
 apiVersion: argoproj.io/v1alpha1
@@ -47,5 +62,5 @@ metadata:
     notifications.argoproj.io/subscribe.<trigger-name>.grafana: tag1|tag2 # list of tags separated with |
 ```
 
-8. Change the annotations settings
-![8](https://user-images.githubusercontent.com/18019529/112022083-47fb0600-8b75-11eb-849b-d25d41925909.png)
+9. Change the annotations settings
+![8](https://user-images.githubusercontent.com/18019529/112022083-47fb0600-8b75-11eb-849b-d25d41925909.png)