Add deprecation notice to PyPI

[bittner]

Hi @yannmh,

we have some issues with Python packages related to @DataDog and Django. If you follow conorbranagan/django-datadog#5 and conorbranagan/django-datadog#7 you can see that there are several developers that struggle with the unclear situation of a package called django-datadog. I think it's worth cleaning things up.

• Could you please add the deprecation notice from this repo's README to PyPI, too?
• Could you maybe ask @conorbranagan in your office to either react to the issues and PRs on GitHub, or update the README with an "unmaintained" notice, and close all PRs and issues?

Thank you for your help,
Peter