Add mumble.proto copyright notice

src/proto/mumble.proto

@@ -1,3 +1,10 @@
+// This is slightly edited src/Mumble.proto from the Mumble repository.
+//
+// Copyright 2009-2021 The Mumble Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style license
+// that can be found in the LICENSE file at the root of the
+// Mumble source tree or at <https://www.mumble.info/LICENSE>.
+
 syntax = "proto2";
 
 option optimize_for = SPEED;