How to install documentation in sub-directory with Package VC?

[Okamsn]

Hello,

I am trying to install a package I wrote using Package VC from Github:
https://github.com/okamsn/loopy/tree/fix-comp-warnings

The lisp files, which are in the root of the project, are installed, but
the documentation file "loopy.texi" is in a subfolder "doc":
https://github.com/okamsn/loopy/tree/fix-comp-warnings/doc

The presence of this file does not trigger the call to makeinfo.
The function `package-vc--unpack-1` seems to look for the keyword ":doc"
in the package description or package spec, which I have tried adding to
the file "loopy-pkg.el" and to the list passed to `package-vc-install`
as `:doc "doc/loopy.texi"`.

What is the correct way to make Package VC install the documentation
contained in the sub-folder?

Thank you.