Update README.md

Author: Witty-Wizard

README.md

@@ -28,3 +28,12 @@ Add the following line to your `platformio.ini` file:
 
 ```ini
 lib_deps = Witty-Wizard/DriveMaster
+```
+
+## License
+
+  <a href="https://www.gnu.org/licenses/gpl-3.0.en.html">
+    <img src="https://www.gnu.org/graphics/gplv3-or-later.svg" alt="GPL‑3.0‑or‑later" width="160">
+  </a>
+
+This library is licensed under the [GNU General Public License v3.0 or later](https://www.gnu.org/licenses/gpl-3.0.en.html).