small documentation and example snippet for non-root wsgi installs

I've found that non-root install as wsgi-script with the new URL_PREFIX aren't very smooth yet.

I've added some snippets to examples and documentation to show what is needed for installation under some path in Apache. Hope this is useful for others. I took my quite some time to find out, why i always got 404 from django.

Could you use "URL-prefixed install" instead of "non-root"?

There's also some mixed indentation in the lines you've added. Other than that, LGTM!