bevy_atmosphere 0.4 for bevy 0.8

Check README.md, docs and examples for usage.

0.4 Change Log

• To change the sky simulation parameters, you would add/update an Atmosphere resource with custom values.
• The plugin doesn't just pick the first camera, but can be used on select cameras using the AtmosphereCamera component, which holds an optional render layer for the spawned skybox to be on.
• The plugin will automatically create skyboxes for atmosphere cameras during the ATMOSPHERE_INIT startup stage, which can be disabled by turning off the "automatic" feature.
• Created skyboxes now have the AtmosphereSkyBox component. Only skyboxes with the component and that have a parent with AtmosphereCamera will have their rotation corrected.
• To change the resolution, you can add an AtmosphereSettings resource and set the resolution field (which should be a multiple of 8). This could be used as part of quality settings in games.

What's Changed

• Relicense bevy_atmosphere under dual MIT/Apache-2.0 by @JonahPlusPlus in https://github.com/JonahPlusPlus/bevy_atmosphere/pull/21

Full Changelog: https://github.com/JonahPlusPlus/bevy_atmosphere/compare/0.3.1...0.4.0