Add tips on preserving file metadata

I have a program that used to overwrite input files in-place, and that was lacking atomicity. When switching to renameio, the biggest question for me was - what `perm` bits should I use? The old `os.OpenFile(path, os.O_WRONLY|os.O_TRUNC, 0)` call didn't need to change any permissions, so it's not obvious what should I do here.

I don't think there's a way to replace a file and keep its permission bits in an atomic way. For now, I'm just using `os.Lstat` first to grab the bits, then using them for `renameio.WriteFile`. I realise that's racy, for example if the permissions change between the two operations, but it seems like an OK tradeoff while keeping it impossible to lose data.

Maybe I'm alone in seeing this as a gap in the README or docs. Do you reckon any tips would be a good addition?

Read the README or existing docs for renameio.WriteFile and compare them with the issue's os.Lstat and perm-bit example. Add guidance explaining how to choose permissions when replacing files and document the metadata-preservation and race considerations described in the issue; done means a user can understand the recommended approach.

Written by the indexing model from the issue text.