Clarify relation to AEM

[kwin]

Currently the readme does only mention AEM once (https://github.com/Netcentric/component-loader/blob/03239e82f3e6413e38fa2fdcdf828aa2f0ad343c/README.md) in

Problem: Mutation Observer vs AEM edit Iframe Postmessage
Solution: Mutation Observer

It would be good to explicitly mention dependencies to AEM (if there are any) and explicitly mention the AEM use case (as that is what this library is primarily used for up till now). In general some introduction would be necessary to outline the goals of this library.

Also the existing headlines in the reade need some refactoring, currently there is Table of contents with children Installation, Usage and Example and API and examples with lots of children, also unrelated ones like Docs, Issue templates and some more which should probably go into a dedicated Contribution section.