Yeah, git clone --recursive is the main thing I suggest. But unlike Google I include the exact command that you can copy and paste into the terminal to clone the specific repo in question.
And if you're reading the README after cloning it already, there are instructions for sorting that out too, also suitable for copy and paste.
Or if you downloadeded the ZIP from GitHub - I'm sorry. But you won't be left too confused, at least you won't if you read my README, because my README covers this situation as well.
(Also: don't forget to git submodule update after changing branch! But if you're noting everything my README tells you, you won't.)
The naive way in this case wouldn’t be to make an issue: How do I clone this repo? I see it has submodules
The naive way would be to just clone the repo without any (apparently) options.
I can attest to this because that’s probably what I would do.
The readme would not resolve a problem that someone knowingly had. It would resolve an unknown upcoming problem.