Add licensing info and new contribution policy

src/main/resources/hotswap-agent.properties

@@ -1,2 +1,20 @@
+#
+# 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/>.
+#
+
 disabledPlugins=Log4j2,Proxy
 LOGGER=info