Document why a copy of kdebindings is needed

[lbeltrame]

Disclaimer: I'm a bioinformatician, but I'm part also of KDE, the community which built kdebindings, which is used by qtbase.

In particular I think it would be nice to know, by a readme or some such:

• Why there's an embedded copy of kdebindings;
• If there have been modification to Smoke or kdebindings locally, as I think it was imported without history.

This is to ensure that in the case of local improvements, whether it would make sense to have them sent to KDE (which would be a good thing).