add more content to readme.md

[mmattel]

@dschmidt can you please add more content to readme.md so that readers and developers can better understand what this repro is about?

[dschmidt]

Yes, certainly. Everything's pretty much WIP which is why I haven't done that yet.
In short: This is a repository for the craft build tool of KDE. So far we use it on Windows only to build the client. See appveyor.yml in the client repository.