Add licensing info and new contribution policy

src/main/kotlin/moe/nea/firmament/features/FirmamentFeature.kt

@@ -1,3 +1,21 @@
+/*
+ * Firmament is a Hypixel Skyblock mod for modern Minecraft versions
+ * Copyright (C) 2023 Linnea Gräf
+ *
+ * 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/>.
+ */
+
 package moe.nea.firmament.features
 
 import moe.nea.firmament.gui.config.ManagedConfig