Specify supported Django versions

[SerhiyRomanov]

I assume we should specify the Django versions that are supported. Also, need to add these to tox.ini

According to Django supported versions policy, it's worth to support 2.2 and 3.0 at that moment.

What do you think about this?

[lukruh]

I have django 3.0 installed and no problems so far, but I only use a subset of the modules functionality for my own.

Yes I think supporting 2.2 and 3.0 seems to be reasonable. I can update the readme and requirements soon.

[lukruh]

feel free to update the tox.ini..