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]

Added a requirements.txt file. Will add to setup.py when released