Vim now handles incremental searching for substitute command natively

[jnpkrn]

Just recently noticed that new behaviour of relatively plain vim,
and tracing it back to when that change got introduced was quite
a procedure incl. hitting this project in web searches.

So it might be worth mentioning in README this very aspect
(basically what "Preview in the command line window" depicts)
got subsumed in vim proper:

vim/vim@b0acacd

Thanks for considering.

[osyo-manga]

hi, Thanks for issues.
Please for pull request!!