Add native GitHub "Sponsor" button linking to Patreon account

[Hexstream]

Simply merge this pull request and activate the "Sponsorships" option in cepl's project settings to get a cute little "Sponsor" button linking to your Patreon account!

Here's a preview of the end result.

Here's the official documentation for this nice feature.

(Note that you do not need to be sponsored by GitHub to use this feature.)