Markdown and video tags

This is a useful way to include a video into Markdown:

[![Watch the video](https://img.youtube.com/vi/T-D1KVIuvjA/maxresdefault.jpg)](https://youtu.be/T-D1KVIuvjA)

The preview image urls are:

https://img.youtube.com/vi/<VIDEO ID>/maxresdefault.jpg
https://img.youtube.com/vi/<VIDEO ID>/hqdefault.jpg

# base URL syntax using:

https://www.youtube.com/watch?v=<VIDEO ID>
https://youtu.be/<VIDEO URL>