Improve README

[louwers]

The README in its current state is in my opinion not a good starting point for users of MapLibre GL Native. Here are some suggestions how I would like to improve it.

• Add a description of what MapLibre GL Native is (apart from being a fork of Mapbox GL Native).
• Move details of how MapLibre was forked to the documentation or to a separate file. I think this information will be less and less relevant as time progresses.
• Add MapLibre's logo.
• Add some screen captures that speak to the imagination.
• Move platform-specific instructions for users of the various SDKs to their respective README files.
• Move instructions relevant for contributors to the core to CONTRIBUTING.md.
• Give some background about the project and current design proposals that are being worked on.

Feedback and other suggestions are welcome.