chore: add license header

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/twincolgrid/TwinColGridListAdapter.java

@@ -1,3 +1,22 @@
+/*-
+ * #%L
+ * TwinColGrid add-on
+ * %%
+ * Copyright (C) 2017 - 2021 Flowing Code
+ * %%
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * #L%
+ */
 package com.flowingcode.vaadin.addons.twincolgrid;
 
 import com.vaadin.flow.component.HasValue;