Update README to mention need for LFS

[dnadales]

Mention in the README that LFS is needed when working with this repo

[ghost]

Could we just remove the need for LFS? Seems like an additional barrier to entry that's not needed, we can just link videos to Google Drive or whatever

[dnadales]

I don't particularly mind. If we do so we might want to add some CI check for broken links. @coot thoughts?

[ghost]

There's a github action that works reasonably well for doing that, I have used it in Hydra.