[Docs] Document best practices for Entitas Redux

[jeffcampbellmakesgames]

Is your feature request related to a problem? Please describe.
It would be good to provide newcomers to either an ECS framework or Entitas Redux with a collection of best practices to better inform how to utilize these approaches or framework in their project.

[matthiashermsen]

Some good examples would be

• Physics?
• Waiting for things before running other systems / logic (async, Coroutines, ..)
• Scene management
• Folder structure
• Communication between ER and Unity (e.g. event listeners)
• Layer abstraction (for testing, but this is less ER related)

[jtuchel]

@jeffcampbellmakesgames that would be great :)