Update documentation for use of this extension

[sugatoray]

Please add some documentation with at least a few figures and a quick GIF demo to make it clear, what is required to use this vscode extension and how to use it.

[nullgit]

Press Ctrl + Shift + P, enter slidev, and click View: Show Slidev

[CatCodeMe]

don't have this option and icon after install 0.4.0

[stefandeml]

you need to select "View: Show Slidev"

[piranna]

I find it easier to have an icon at the top-right of the editor when detecting a Slidev document... Maybe it can be done if it's a .md file and has a frontmatter.

[piranna]

But yeah, at least add this info about how to enable it to the readme.

[piranna]

In any case, the extension is not working for me either.

[Fanisting]

In any case, the extension is not working for me neither.

[Giuseppe499]

It is not working also for me