Add GPL3 header

src/main.rs

@@ -1,3 +1,20 @@
+/* Gitm. Automatic git mirroring script.
+ *
+ * Copyright (C) 2021 Arsen Musayelyan <arsen@arsenm.dev>
+ *
+ * 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/>.
+ */
+
 use std::{collections::HashMap, env, process::{Command, Child, exit}};
 use serde_derive::Deserialize;
 use std::fs;