Add copyright notice

Author: mrapplexz

src/commonMain/kotlin/net/pearx/okservable/collection/ObservableMap.kt

@@ -1,3 +1,10 @@
+/*
+ * Copyright © 2020, PearX Team
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
 @file:Suppress("UNCHECKED_CAST")
 
 package net.pearx.okservable.collection