docs(README): Adding webhook description

Signed-off-by: Vincent Boutour <[email protected]>

Author: ViBiOh

README.md

@@ -56,6 +56,36 @@ It can be read-only or with edit right. With edit-right, user can do anything as
 
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+### Webhook
+
+You can register webhook listeners on folders and receive an HTTP notification when one of these event occurs:
+
+- `create` occurs when a directory is created
+- `upload` occurs when an item is uploaded
+- `rename` occurs when an item is renamed
+- `delete` occurs when an item is deleted
+- `start` occurs when fibr start and do something on an item
+- `access` occurs when content is accessed (directory browsing or just one file)
+
+The request sends is a POST with 15s timeout with the given payload structure:
+
+```json
+{
+  "item": {
+    "date": "2021-08-10T19:31:28.952325533Z",
+    "name": "payload.json",
+    "pathname": "/path/to/payload.json",
+    "isDir": false,
+    "size": 177
+  },
+  "type": "upload"
+}
+```
+
+It will contains an extra key `new` with the same structure of `item` in case of a `rename` event.
+
+The webhook can be recursive (all children folders will be notified too) and for now, no event type selection is possible: the webhook will receive all kinds of events.
+
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