Create FUNDING.yml

* Displaying a sponsor button in your repository, read documentation at https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/displaying-a-sponsor-button-in-your-repository
* Using the OpenCollective service, see https://opencollective.com/xwiki

.github/FUNDING.yml

+# You can add one username per supported platform and one custom link
+open_collective: xwiki