Declare explicit dependency on django-chartflo

[brylie]

As was discussed in the django-chartflo roadmap issue, this package should remain separate from django-chartflo. In order to use django-chartflo as a dependency, this chart should explicitly reference django-chartflo (e.g. in a requirements.txt).

Add a requirements.txt, or the equivalent for PIP packaging, that references django-chartflo as a dependency of this project.

[synw]

I added a requirements.txt file. The setup.py is here but not used yet as django-chartflo was not yet packaged. I must admit that this team of modules got a bit neglected since a while. Time to update all this and make it more usable

[synw]

It looks like some Django 2 checks has to be made as well to ensure compatibility #2 . Working on it