Clarify Warning message on readme

[troygrosfield]

The warning message on the readme makes it sound like this project is no longer being maintained or needed (at all) by django apps running celery. I'm not sure if that's the case either way, but the readme should clear up the wording a bit.

If this project is still actively maintained then maybe something along the lines of:

With the release of celery x.y.z, this lib is no longer required for celery integration into your django apps. However, this repo is still actively maintained and provides the following benefits beyond what celery provides: <list of awesomeness>

[zachborboa]

related https://github.com/celery/django-celery/issues/335 https://github.com/celery/celery/issues/2174

[zachborboa]

I was also hoping the project's description Old Celery integration project for Django would be updated to something like Optional Celery integration project for Django.

[brianmay]

Also see #410 and #391.