add documentation

docs/tutorials/webhook-provider.md

@@ -26,6 +26,8 @@ The following table represents the methods to implement mapped to their HTTP met
 | AdjustEndpoints | POST        | /adjustendpoints | Provider specific adjustments of records |
 | ApplyChanges    | POST        | /records         | Apply record                             |
 
+OpenAPI spec is [here](../../api/webhook.yaml).
+
 ExternalDNS will also make requests to the `/` endpoint for negotiation and for deserialization of the `DomainFilter`.
 
 The server needs to respond to those requests by reading the `Accept` header and responding with a corresponding `Content-Type` header specifying the supported media type format and version.