Add GitHub sponsors button to feature liberapay

[ghost]

• I carefully read the contribution guidelines and agree to them.

Changes proposed in this request:

• Adds the GitHub sponsors button. Here's an example with playnite:
  
  https://github.com/JosefNemec/Playnite
  Info from GitHub on this below:

.github/FUNDING.yml shows the community how to support this project. Please see our repository funding links [documentation ](https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository) for more information on formatting and what is and isn't allowed in this file.

Please note that funding links are currently disabled on this repository. Visit repository settings to enable display of your funding links.

[ghost]

@theScrabi please note that this needs to be enabled from the repository settings