Update README with instructions for Lazy >= v11

[kevinm6]

This PR contains the instructions to make image.nvim working with Lazy v11 and above, as discussed here.

@3rd I didn’t renamed the rockspec leaving that to you, even because maybe you want to consolidate that and make it to update itself via GH Actions since just the git-ref should be update and maybe one more variable.

[kevinm6]

I think that maybe (if they keep as is) could even not been require after this commit in Lazy

[kevinm6]

Closed since #236