Update readme.md with additional feeder link, new header for ads-b derived datasets, small tweaks

[MrAirspace]

Hi Rik, Please let me know your thoughts. Im not sure whether to create a new header for the ads-b derived datasets or whether it's better to place it under the 'software' header. For now I went for this approach. Please advise what you prefer, then I could also change it if better. Thanks and best, Sebastiaan.