Add a funding-manifest-urls file, as required by the floss.fund funding
manifest:

> If the manifest references URLs, such as a project's webpage, that are not
> on the same domain as the funding.json manifest itself, then the wellKnown
> URL must point to a /.well-known/funding-manifest-urls file. This file should
> contain URL (one or more) of the funding.json manifest URLs that reference
> it. This establishes provenance and verifies that that the publisher of the
> manifest is authorised to solicit funding on behalf of the URLs (entity,
> projects) described in the manifest. This follows the .well-known convention.

Related: https://floss.fund/funding-manifest/
Related: postmarketos.org MR 345