Mention Github Signature Verification Issue

[lukas-mika-maersk]

Description

I'd like to add the information, that Github Signature Verification of Keyless Signatures is currently not supported, to page "Detailed Gitsign Usage".

The paragraph would also link to the respective Github Issue, so that interested users can check for the latest status of this feature. This might even help to raise visibility of the issue, since users can upvote this ticket.

[smythp]

Are you planning on doing a PR for this? I can tackle it, but I also encourage you to do a PR, or to share some proposed text here in the issue. Thank you!