It’s not possible to embed videos directly, but you can put an image which links to a YouTube video:
[![IMAGE ALT TEXT HERE](https://img.youtube.com/vi/YOUTUBE_VIDEO_ID_HERE/0.jpg)](https://www.youtube.com/watch?v=YOUTUBE_VIDEO_ID_HERE)
- For more information about Markdown look at this Markdown cheatsheet on GitHub.
- For more information about Youtube image links look this question.