Add a funding / support badge

[philippgille]

Patreon, Flattr, Gratipay / Liberapay, ...

See https://alternativeto.net/software/patreon/

Or Bountysource, where people can pay to get a specific issue solved.

[philippgille]

Gratipay is discontinued, the final blog post recommends as alternatives:

• https://www.patreon.com/
• https://liberapay.com/ (Gratipay fork)
• https://opencollective.com/

Bountysource is different and should be included as well.