Add a note directing users to platformdirs

[pradyunsg]

https://pypi.org/project/platformdirs/ is a friendly fork, that seems to be much better maintained nowadays.

It would be useful to direct users their way, especially if there aren't maintainance resources available for this project (which seems to be the case based on #79).

[machow]

After evaluating both, and reading the issue below I have to admit that I decided to depend on appdirs (edit: and then likely shift to platformdirs as an appdirs "v2"). I need stronger guarantees about stability of what gets returned (vs correctness). (Still thinking about this though, and appreciate all the time everyone has put into these libraries).

• Document changes to MacOS's configuration paths, compared to last appdirs release tox-dev/platformdirs#47,

[pradyunsg]

The problem there isn't that there were behaviour changes in a minor release (I misread things) but that there's a behaviour change/bugfix compared to what appdirs does. I've updated the issue to reflect that.

Anyway, you're free to use an abandoned-but-functional package. I'm not policing what you do in your projects. :)

[pradyunsg]

Thanks @zoofood! ^.^