Update README: add improved undojoin solution.

[bachhh]

@hoschi proposed a nice fix for issue #134, I hope it can be put on the front page, in README.md.

[hoschi]

Is this still needed? I don't have it in my vimrc anymore. 🤔

[bachhh]

I still get this issue on Neovim 5.0.
But this surprised me a bit, when an author doesn't use their own plugin anymore. Did you find something better ? 🤔

[hoschi]

I use Neoformat every day, I just don't have the fix from #134 anymore in my vimrc. The problem is four years old, with my comment I just wanted to question if it is still a problem. I'm not the author of Neoformat.

[bachhh]

Very sorry that i misread your Github handle, my brain wasn't working well this afternoon, that was very embarrassing of me.
Yeah I still got that strange undo bug on Nvim 0.5.0, did you not also get this ?

[hoschi]

Ok I found whey I don't need the fix anymore. I created a shortcut to write files a long time ago, this one decides what happens on a "per project" basis and handles formatting as well. Therefore, I don't need to fix the original problem and don't have the fix in vimrc anymore.

I tested #134 again and it still occurs, so the PR is totally valid.