server: API support for webhooks

Closes #339

doc/API.md

@@ -13,6 +13,7 @@
    - [Error handling](#error-handling)
    - [Field selecting](#field-selecting)
    - [Versioning](#versioning)
+   - [Webhooks](#webhooks)
 
 2. [API reference](#api-reference)
 
@@ -276,6 +277,15 @@ reject the request as well, in which case the client is encouraged to notify
 the user about the situation.
 
 
+## Webhooks
+
+System administrators can choose to configure webhooks to track events.
+Webhook URIs can be configured in `config.yaml` (See `config.yaml.dist` for
+example). Upon any event, the API will send a `POST` request to the listed
+URIs with a [snapshot resource](#snapshot) generated with anonymous user
+privileges as the message body, in JSON format.
+
+
 # API reference
 
 Depending on the deployment, the URLs might be relative to some base path such