Document supported operating systems

[FeldrinH]

The GitHub repo summary says that this is for 'Linux and Windows', but the docs don't seem to mention any specific operating systems and a look through old issues seems to indicate at least some level of support on Mac.

It would be useful to state explicitly in the documentation what operating systems are supported.

[alshdavid]

I know this isn't documentation, but I have tried this library on MacOS, Windows and Linux without issue.

+1 on more comprehensive documentation though

[facetious]

This library doesn't work on Mac without issue since it uses O_EXCL when creating. See https://stackoverflow.com/a/73752984/2708650 for more details about why this shouldn't be done.