Add GPLv3 license notice

Program.cs

@@ -1,4 +1,22 @@
-using Serilog;
+/*
+ * csSiteGen - A static site generator written in c#
+ * Copyright © 2022 Robert Morrison<sherlock5512>
+ *
+ * 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 <http://www.gnu.org/licenses/>.
+ */
+
+using Serilog;
 using System.Reflection;
 
 namespace csSiteGen;