Adding: Github sponsor button

[rskikuli]

We can now add a 'sponsor' button in our repositories to increase the visibility of funding options for CityZen.
More funding will help us have more visibility, cover some of the costs (domain etc) and also print some promotional materials to start with.
Here are the details about this feature form Github: https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository
If anyone from the team wants to take this over let me know.