Indicate that REQUIRE_BASE_URL is relative.

When I was new to this library, I was confused about what to specify as a `REQUIRE_BASE_URL`. I've tried both a filesystem path to my django app's static dir e.g. `/path/to/myproject/myapp/static` and an absolute URL to static assets e.g. `/static/`. But they didn't work fine. After seeing [a topic in the forum](https://groups.google.com/forum/#!topic/django-require/kVUgulN4uf0), I realized that there should be a **relative** path.

As my English is not so good, please feel free to close this pull request and rewrite by yourself.