mention compatibility with xdist

[ssbarnea]

As I do know that plugins that change order are known ti cause conflicts with use of pytest-xdist, it would be a good idea to mention the status of it, if is supported or not.

If it is supported it would be a good idea to also test to prevent regresisons.

[jbwdevries]

I've added a note about how this plugin interacts with xdist.

Unfortunately, to really get it to work, I would have to dig in into xdist's schedulers; I don't think I can update the tests send out to nodes afterwards, and I'm having a hard time figuring out if I can send out the tests in batches. Also, it doesn't seem the schedulers are very composable, so if you'd be using a different scheduler from another plugin, it'd stop working.