Plugins: update the doc and add a changelog bragline about 'invoice_payment' notification

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - build: now requires `python3-mako` to be installed, i.e. `sudo apt-get install python3-mako`
+- plugins: a new notification type `invoice_payment` (sent when an invoice is paid) has been added
 
 ### Deprecated
 

doc/PLUGINS.md

@@ -223,6 +223,21 @@ to a peer was lost.
 }
 ```
 
+#### `invoice_payment`
+
+A notification for topic `invoice_payment` is sent every time an invoie is paid.
+
+```json
+{
+	"invoice_payment": {
+		"label": "unique-label-for-invoice",
+		"preimage": "0000000000000000000000000000000000000000000000000000000000000000",
+		"msat": "10000msat"
+	}
+}
+```
+
+
 #### `warning`
 
 A notification for topic `warning` is sent every time a new `BROKEN`