docs: document "notify_klippy_ready" websocket notfication

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

docs/api_changes.md

@@ -18,6 +18,8 @@ facing APIs.
   Currenlty subscriptions are only supported over the a
   websocket connection, one may use the id received from
   `server.websocket.id`.
+- The `notify_klippy_ready` websocket notification has been
+  added.
 
 ### November 2nd 2020
 - The `GET /server/files/directory` endpoint now accepts a new

docs/web_api.md

@@ -816,6 +816,11 @@ Status Subscriptions arrive as a "notify_status_update" notification:
 The structure of the status data is identical to the structure that is
 returned from an object query's "status" attribute.
 
+### Klippy Ready:
+Notify clients when Klippy has reported a ready state
+
+`{jsonrpc: "2.0", method: "notify_klippy_ready"}`
+
 ### Klippy Disconnected:
 Notify clients when Moonraker's connection to Klippy has terminated