document how to add a dropin file with Hiera

admin-guide/configuration/services.md

@@ -13,6 +13,31 @@ base::services:
 ```
 The key inside is the `systemd` service name without the `.service` suffix.
 
+## Extend/Override Unit File Entries with a Dropin
+You can create a dropin file by setting `dropin` to `true` and add the content of the dropin unit file below the `options` key, as in following examples:
+```
+base::services:
+    netdata:
+        enable: true
+        dropin: true
+        options:
+            Service:
+                Environment: "FOO=bar"
+```
+or if there is more than one value for the same key:
+```
+base::services:
+    netdata:
+        enable: true
+        dropin: true
+        options:
+            Service:
+                Environment:
+                - "FOO=bar"
+                - "BIZ=buz"
+```
+
+
 ## Disabling/Stopping a Service
 
 To stop and disable an already running service, disable it in the `base::services` Hiera key with `enable: false`: