Add docs to custom vertex attribute example

[ChristopherBiscardi]

Objective

The gltf-json crate seems like it strips/adds an _ when doing the name comparison for custom vertex attributes.

• gltf-json add
• gltf-json strip
• bevy's handling seems like it uses the non-underscore'd version.

The bevy example gltf: barycentric.gltf, includes two underscores: __BARYCENTRIC in the gltf file, resulting in needing _BARYCENTRIC (one underscore) as the attribute name in Bevy. This extra underscore is redundant and does not appear if exporting from blender, which only requires a single underscore to trigger the attribute export.

I'm not sure if we want to change the example itself (maybe there's a reason it has two underscores, I couldn't find a reason), but a docs comment would help.

Solution

add docs detailing the behavior