docs(shared): Include assets instructions (#592)

* docs(shared): Include assets instructions

* doc: fix typo

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Co-authored-by: Alvaro Saburido <alvaro.saburido@gmail.com>

CONTRIBUTING.md

@@ -97,4 +97,8 @@ Adding a new third party library is generally discouraged, unless it is absolute
 
 ## Keep core small
 
-The core package should be as small as possible, it should only contain the core functionality of the library. If you are adding a new feature, please consider adding it as a plugin instead. for example, if you want to add support for [Effect Composer](https://threejs.org/examples/?q=compo#webgl_postprocessing_effectcomposer) you should create a new package called `@tresjs/post-processing` and add it as a plugin. If it's a smaller scope you can always add it to the `cientos` package.
+The core package should be as small as possible, it should only contain the core functionality of the library. If you are adding a new feature, please consider adding it as a plugin instead. for example, if you want to add support for [Effect Composer](https://threejs.org/examples/?q=compo#webgl_postprocessing_effectcomposer) you should create a new package called `@tresjs/post-processing` and add it as a plugin. If it's a smaller scope you can always add it to the `cientos` package.
+
+### Assets
+
+If you need/want to add assets like models, videos, musics, textures, etc. Please consider adding to our [official assets repo](https://github.com/Tresjs/assets).