Add README

Add some README with basic information and relevant links.

This is considered a work in progress and contributions plus feedback are very welcome! For now I copied most of the text from the Spectacle README, so there might also be some wrong information in there.

In general, I was thinking about unifying the READMEs of the KDE projects here and try to reduce redundant information, so when things change information only needs to be changed in one place while the projects are just linking there.