Document Temp Dir Creation

[svozza]

**Is your feature request related to a problem?
I recently ran into the the issue desribed in #329, this caused me to waste a significant amount of time trying to figure out why my tests were passing on my Mac but not on my build system running Ubuntu. I assumed the file system was blank and was creating the /tmp directory in my tests and of course this worked fine locally because the temp directory pyfakefs creates for MacOS is /var.

Describe the solution you'd like
Document that the filesystem is not blank and explain the difference between MacOs and Linux.

Describe alternatives you've considered
n/a

I'm willing to make the PR to if someone can tell me where this note should live.

[mrbean-bremen]

Thanks - good point! That could definitely help other users.
I think this belongs somewhere in the usage chapter, though I'm not sure where exactly - probably add a subtopic for troubleshooting? Maybe we can think of other problems that have occurred, for example problems with patching some libraries.
We would definitely appreciate a PR for this!

[svozza]

Sounds good, I should be able to stay this tomorrow!