docs: Document "notify_klippy_shutdown"

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

README.md

@@ -21,4 +21,4 @@ Users:\
 [user_changes.md](/docs/user_changes.md) - December 31st 2020
 
 Developers:\
-[api_changes.md](/docs/api_changes.md) - December 30th 2020
+[api_changes.md](/docs/api_changes.md) - January 1st 2021

docs/api_changes.md

@@ -1,4 +1,7 @@
 This document keeps a record of all changes to Moonraker's web APIs.
+### January 1st 2021
+- A `notify_klippy_shutdown` websocket notification has been added
+
 ### December 30th 2020
 - Some additional fields are now reported in the response to
   `GET /machine/update/status`.

docs/web_api.md

@@ -1104,6 +1104,11 @@ Notify clients when Klippy has reported a ready state
 
 `{jsonrpc: "2.0", method: "notify_klippy_ready"}`
 
+### Klippy Shutdown:
+Notify clients when Klippy has reported a shutdown state
+
+`{jsonrpc: "2.0", method: "notify_klippy_shutdown"}`
+
 ### Klippy Disconnected:
 Notify clients when Moonraker's connection to Klippy has terminated