Add license to readme

Author: vojtech-stehlik

README.md

@@ -70,4 +70,23 @@ $id = $api->getImageFromFile(
 $map = (FilledMap::get())->setMapId($id);
 /** @var \pocketmine\player\Player $player */
 $player->getInventory()->addItem($map);
+```
+
+## 💡 License
+```
+ImageOnMap - Easy to use PocketMine plugin, which allows loading images on maps
+Copyright (C) 2021 CzechPMDevs
+
+This program is 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.
+
+This program 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 <https://www.gnu.org/licenses/>.
 ```