Mention FFmpeg license on website

website/src/components/_Download.md

@@ -6,4 +6,6 @@
 🌱 **This is the _beta-17_ release** 🌱<br/>
 Coollab is already usable and powerful, but many things are still to be improved.<br/>
 Please share us your feedback [on our Discord](https://discord.gg/QEjqnEy4aT) or at https://github.com/CoolLibs/Lab/issues/new/choose.
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+*This software uses code of [FFmpeg](http://ffmpeg.org) licensed under the [GPLv3.0](https://www.gnu.org/licenses/gpl-3.0.html) and its source can be downloaded [here](https://github.com/CoolLibs/Lab).*