doc: add submodule policy in CONTRIBUTING.md

[monperrus]

No description provided.

[sonatype-lift]

⚠️ 85 God Classes were detected by Lift in this project. Visit the Lift web console for more details.

[slarse]

I think it'd be appropriate to at least mention that we don't publish submodules?

[monperrus]

agree. merged suggestion.

[slarse]

This is good for me.

@SirYwell @I-Al-Istannen @MartinWitt anything to add? I think the CONTRIBUTING.md file is pretty important so I don't want to merge without you getting the chance to say your piece.

[MartinWitt]

I thought about adding a hint for jitpack.io, but sadly this is pretty unstable and no one should use this for production code. So lgtm you can merge.

[I-Al-Istannen]

I have found jitpack to be quite reliable for other projects, but users can probably find something nice on their own here.

[slarse]

I also think that Jitpack is a little bit beyond the scope of these docs (although I concur with @I-Al-Istannen in that I've found it to be reliable when I've used it).

Merging this now so we get it in, we can always update it later!

[slarse]

Thanks @monperrus