Feature request ⚡: Update README with optional requirements

[gorillamoe]

Update the README to inform about optional requirements that make the overall experience of using Snap.nvim (hopefully) much nicer.

One of these optional requirements would be:

• juu.nvim -- which makes notifications and progress on download and un-archive looks really nice

It's not strictly necessary to have use it, as the default vim.notify works just fine, but for people like me who are suckers for visuals, that might be a nice addition.