well-known/funding-manifest-urls: new file

(Trying to finally finish up postmarketos.org!345 (merged).)

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!345 (merged)