Add short demo for new users

[shakesbeare]

Adds demonstration video as requested in #41.

I had a guess at the commands I thought would be most commonly used to show off in a demo that's just over 30s.

[epwalsh]

This is great! Thank you

[vimpostor]

Looks like you uploaded the demo video to a non-permanent address, because the video in the README is broken.
I also don't see the video in your fork's commit history anymore, which is why Github may have garbage collected the video.

Edit: Never mind, it works now, my browser just doesn't support the video format for some strange reason, but wget https://user-images.githubusercontent.com/75107188/227362168-29ff9d4d-5b62-4aff-9442-21cd8c072d29.mp4 worked.

[epwalsh]

Hm, it looks like something weird has happened. It doesn't render like it used to for me.

[epwalsh]

I think I may have fixed it.