Add Zlib license.

yt-conv

@@ -1,5 +1,27 @@
 #!/bin/bash
 
+# Copyright (C) 2025 Nicolás Ortega Froysa <nicolas@ortegas.org> All rights reserved.
+# Author: Nicolás Ortega Froysa <nicolas@ortegas.org>
+#
+# This software is provided 'as-is', without any express or implied
+# warranty. In no event will the authors be held liable for any damages
+# arising from the use of this software.
+#
+# Permission is granted to anyone to use this software for any purpose,
+# including commercial applications, and to alter it and redistribute it
+# freely, subject to the following restrictions:
+#
+# 1. The origin of this software must not be misrepresented; you must not
+#    claim that you wrote the original software. If you use this software
+#    in a product, an acknowledgment in the product documentation would be
+#    appreciated but is not required.
+#
+# 2. Altered source versions must be plainly marked as such, and must not be
+#    misrepresented as being the original software.
+#
+# 3. This notice may not be removed or altered from any source
+#    distribution.
+
 if [ $# -ne 1 ]
 then
 	>&2 echo "Usage: $0 <url>"