Add license header

src/main/java/me/clip/placeholderapi/util/Futures.java

@@ -1,3 +1,23 @@
+/*
+ * This file is part of PlaceholderAPI
+ *
+ * PlaceholderAPI
+ * Copyright (c) 2015 - 2020 PlaceholderAPI Team
+ *
+ * PlaceholderAPI free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * PlaceholderAPI is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
 package me.clip.placeholderapi.util;
 
 import org.bukkit.Bukkit;