docs: add License section to README

Author: velut

README.md

@@ -1,2 +1,22 @@
-# extractor
+# @jsdocs-io/extractor
+
 API extractor for npm packages
+
+## License
+
+    AGPL-3.0-or-later
+
+Copyright (C) 2021 Edoardo Scibona
+
+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/>.