docs: document new "notify_metadata_update" notification

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/api_changes.md

@@ -1,7 +1,40 @@
 This document keeps a record of all changes to Moonraker's remote
 facing APIs.
 
-### August 16 2020
+### September 1st 2020
+- A new notification has been added: `notify_metdata_update`.  This
+  notification is sent when Moonraker parses metdata from a new upload.
+  Note that the upload must be made via the API, files manually (using
+  SAMBA, SCP, etc) do not trigger a notification.  The notification is
+  sent in the following format:
+  ```
+  {jsonrpc: "2.0", method: "notify_metadata_update", params: [metadata]}
+  ```
+  Where `metadata` is an object in the following format:
+
+  ```json
+  {
+    filename: "file name",
+    size: <file size>,
+    modified: "last modified date",
+    slicer: "Slicer Name",
+    first_layer_height: <in mm>,
+    layer_height: <in mm>,
+    object_height: <in mm>,
+    estimated_time: <time in seconds>,
+    filament_total: <in mm>,
+    thumbnails: [
+      {
+        width: <in pixels>,
+        height: <in pixels>,
+        size: <length of string>,
+        data: <base64 string>
+      }, ...
+    ]
+  }
+  ```
+
+### August 16th 2020
 - The structure of data returned from `/printer/info` (`get_printer_info`)
   has changed to the following format:
   ```json

docs/web_api.md

@@ -747,6 +747,36 @@ The following `actions` are currently available:
 - `move_item`
 - `copy_item`
 
+### Metadata Update
+When a new file is uploaded via the API a websocket notification is broadcast
+to all connected clients after parsing is complete:
+
+`{jsonrpc: "2.0", method: "notify_metadata_update", params: [metadata]}`
+
+Where `metadata` is an object in the following format:
+
+```json
+{
+  filename: "file name",
+  size: <file size>,
+  modified: "last modified date",
+  slicer: "Slicer Name",
+  first_layer_height: <in mm>,
+  layer_height: <in mm>,
+  object_height: <in mm>,
+  estimated_time: <time in seconds>,
+  filament_total: <in mm>,
+  thumbnails: [
+    {
+      width: <in pixels>,
+      height: <in pixels>,
+      size: <length of string>,
+      data: <base64 string>
+    }, ...
+  ]
+}
+```
+
 # Appendix
 
 ### Websocket setup