Docs: Add community plugins to README

[gkjohnson]

Added community plugins section with links and descriptions to three/plugins/README.md.

cc @WilliamLiu-1997 @bertt - I've added a new list of "Community Plugins" and included both of your projects for gaussian splats and the styling spec.

cc @jo-chemla @dbuck @AnthonyGlt @sguimmara @shotamatsuda in case you guys are aware of any other plugins that might be good to include in this list.

[bertt]

https://github.com/bertt/3dtilesrenderer-outline-plugin

[jo-chemla]

Hi Garrett, thanks for the ping, but I don't know of existing community plugins for 3DTilesRendererJS - nice seeing this new community-plugins section, just made me discovered the impressive WilliamLiu-1997/3D-Tiles-RendererJS-3DGS-Plugin, love seeing a sparksjs based OGC-3Dtiles-of-3dgs-packed-in-glb extension!

Do you think it could make sense to add a similar section for 3DTilesRendererJS integrations - not plugins, but eg

• the great work by shotamatsuda in takram-design-engineering/three-geospatial
• maplibre current 3Dtiles example that is probably going to evolve into a maplibre plugin
• Giro3D integration starting v0.42
• threepipe plugin-3d-tiles-renderer
• potree + 3D-tiles PR Integration of Google 3D Tiles with Potree using NASA AMMOS 3D Tiles Renderer potree/potree#1408

[gkjohnson]

I'd be open to a "community resources" section or something if you'd like to make a PR.

[jo-chemla]

Sure I can do that quick PR. I would imaging this Communtiy Resources being placed within the main README.md of the repo, rather than the threejs/threejs-plugins readmes, right?

[gkjohnson]

placed within the main README.md of the repo, rather than the threejs/threejs-plugins readmes, right

Sure we can start with this - if it gets too long we can move it to something like a RESOURCES.md file. It would be helpful to have some consistent notion of what a "community resource" is, as well, if you have any kind of definition.

[jo-chemla]

Alright, just did so at this PR, please don't hesitate if you need any edits!

#1590