Add Asset Library Submission Guidelines

[aaronfranke]

This PR contains a handful of requirements and recommendations that I've come up with so far. Formalizing these will help both users and moderators with the submission process.

I've placed them in the "Submitting to the Asset Library" document, above the "Submitting" section, since it's best for users to check if their asset meets the requirements before they begin submitting.

It might be worth linking to this page from the "Submit Asset" form on the asset library.

I also added a little bit to the "About the Asset Library" page. It's not really related to the above, but it's something that should be included as well, so I just put it in here.

[Calinou]

I think anyone can upload an asset in the "Demos" category. Only select assets will have the "Official" support level though.

[Calinou]

Looks good to me, thanks for taking the time to write some guidelines 🙂

[YuriSizov]

Can we recommend adding a .gitattributes file to have cleaner asset installs? Something like

/.gitattributes     export-ignore
/.gitignore         export-ignore
/LICENSE            export-ignore
/README.md          export-ignore
/project.godot      export-ignore

[Calinou]

@pycbouh That makes sense, though we need to tell asset authors to make a second copy of the license in the add-on folder. Otherwise, the asset could become detached from its license if you commit it to version control (which is common to avoid using submodules).

Unfortunately, one can't have just a single copy of the license in the add-on folder as GitHub won't be able to pick up the repository license that way.

[YuriSizov]

@Calinou Worth adding this to recommendations as well, then. This can be solved in future by actually using repository root as res://addons/addon_name root, so that everything that installs is already "sandboxed".

[aaronfranke]

@pycbouh See godotengine/godot-proposals#554, I think it would be better if assets could be extracted into subfolders. I think it's best to preserve the README/etc files, but we just don't want them in the root of the project folder.

But also, some assets are indeed full projects, so we can't ignore the project.godot file for them.

[YuriSizov]

@aaronfranke Obviously, there are different kinds of addons and this blacklisting thing should be noted in docs with a remark.

Yet, I feel like it should be noted nonetheless, as whitelisting/blacklisting files is a common question when publishing modules, which addons essentially are. And currently this is the only way for plugin developers to avoid polluting end-users' projects. Your proposal obviously supersedes this suggestion, but it's not something we can change short-term.

[aaronfranke]

Adding such a note can be done in another PR, if desired. In the meantime, I'll merge this.