docs: add upgrade event in the documentation

Reference: https://github.com/socketio/engine.io/blob/898bd1c9df379f4749989afe82c311b7ed9c6174/lib/socket.ts#L285

Related: socketio/socket.io-website#369

README.md

@@ -338,6 +338,14 @@ A representation of a client. _Inherits from EventEmitter_.
     - Fired when an error occurs.
     - **Arguments**
       - `Error`: error object
+- `upgrading`
+    - Fired when the client starts the upgrade to a better transport like WebSocket.
+    - **Arguments**
+        - `Object`: the transport
+- `upgrade`
+    - Fired when the client completes the upgrade to a better transport like WebSocket.
+    - **Arguments**
+        - `Object`: the transport
 - `flush`
     - Called when the write buffer is being flushed.
     - **Arguments**