Readme and developer documentation

[argallegos]

• Design the readme in a style similar to the cesium-unreal readme.
• Create developer documentation to help people get started contributing to cesium-o3de.

[argallegos]

@shehzan10, please take a look at the readme here.

Tutorial links and download instructions, as well as the images, are still in progress, but some sections need your review for wording. In particular:

Cesium for O3DE and the 3D Geospatial Ecosystem. This section is exactly the same as the section on cesium-unreal, with Unreal replaced with O3DE. Does this look good, or does anything else need to be changed?

O3DE Integration. This is currently just one sentence. The corresponding paragraph in the cesium-unreal readme says, "Cesium for Unreal is tightly integrated with Unreal Engine making it possible to visualize and interact with real-world content in editor and at runtime. The plugin also has support for Unreal Engine physics, collisions, character interaction, and landscaping tools. Leverage decades worth of cutting-edge advancements in Unreal Engine and geospatial to create cohesive, interactive, and realistic simulations and applications with Cesium for Unreal."

Most of those features don't really carry over to O3DE. What do you recommend we say instead?

[baothientran]

I've just added the release guide in the Documentation directory

[baothientran]

Added placeholder for CHANGES.md

[shehzan10]

@baothientran @argallegos All looks good to me. Please take one last pass to check for typos etc and then merge it in.