add license headers

common/addons/language-yaml/src/main/java/com/dfsek/terra/addons/yaml/YamlAddon.java

@@ -1,3 +1,10 @@
+/*
+ * Copyright (c) 2020-2021 Polyhedral Development
+ *
+ * The Terra Core Addons are licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in this module's root directory.
+ */
+
 package com.dfsek.terra.addons.yaml;
 
 import com.dfsek.tectonic.yaml.YamlConfiguration;