docs: add GPL v3 license and GitHub source link

LICENSE

@@ -0,0 +1,14 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The GNU General Public License is a free, copyleft license for
+software and other kinds of works.
+
+  For the precise terms and conditions for copying, distribution and
+modification, see https://www.gnu.org/licenses/gpl-3.0.txt

README.md

@@ -1,5 +1,9 @@
 # 8-cut
 
+[![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://github.com/ethanfel/8-cut/blob/master/LICENSE)
+
+**Source:** https://github.com/ethanfel/8-cut
+
 A desktop tool for cutting 8-second clips from video files, designed for building [SELVA](https://github.com/google-deepmind/selva) datasets.
 
 ## Overview
@@ -95,3 +99,7 @@ pytest tests/ -v
 ```
 
 38 unit tests covering path builders, ffmpeg command generation, time formatting, and the processed-clips database.
+
+## License
+
+[GNU General Public License v3.0](https://github.com/ethanfel/8-cut/blob/master/LICENSE)