license: add GPL license header

Signed-off-by: Rui Chen <rui@chenrui.dev>

README.md

@@ -241,4 +241,11 @@ For now I am focussing more on [planned features](TODO.md) for a **stable** v1.0
 
 ## License
 
-[LICENSE](LICENSE)
+Copyright (C) 2022 mr-karan
+
+This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero 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 Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along with this program. If not, see <https://www.gnu.org/licenses/>.
+