Add docs - Things to think about for performance

[grofit]

Could do with adding docs that cover how to get more performance out of the lib covering things like:

• Using component type ids vs types
• Segregating collections with system affinities
• Presetting the default sizes/expansions of certain objects
• Using ReactToGroup over other systems if you are just batch processing
• Using structs for pure value type data
• Prefetching components if needed

[grofit]

I have started adding some of the documentation to this section so will close this for now, as always, anyone can raise a new issue around adding of new docs etc.