docs: add notification-api-v1 spec

README.md

@@ -54,4 +54,11 @@ This returns a JWT access token required to provided as Bearer Token for any req
 2. Request the all available reports: `GET /reports` (see OpenAPI specs for details)
 3. Trigger the calculation of a reports data: `POST /report-calculation/<report-id>`
 4. Get status of the report calculation: `GET /report-calculation/<calculation-id>`
-5. Once the status shows the calculation is completed, get the actual result data: `GET /report-calculation/<calculation-id>/data`
+5. Once the status shows the calculation is completed, get the actual result data: `GET /report-calculation/<calculation-id>/data`
+
+
+## Subscribe to continuous changes of a report
+1. Create an initial webhook (if not already registered): `POST /webhook`
+2. Register for events of the selected report: `POST /webhook/{webhookId}/subscribe/report/{reportId}:`
+3. You will receive Event objects to your webhook, including an initial event directly after you subscribe, pointing to the current report data
+4. Use the report-calculation-id in the event to fetch actual data from /report-calculation endpoint