Give more context to FOSTERplus in the main README

[lambo]

E.g. give the Horizon 2020 funding number in a separate paragraph somewehere at the end of the page.

Introduce in 1-2 sentences why we had the "book sprint" working package in FOSTERplus in the first place, and a link to some meaningful, "official" overview on who FOSTERplus is, and what it is doing.