Clean up the readme
Clean up the readme

All user docs have been moved to https://github.com/gtk-rs/gtk-rs.github.io/tree/master/docs-src

[skip ci]

daa84/gtk,gkoz/gtk-rs-gtk,Susurrus/gtk,gkoz/gtk-rs-gtk,gkoz/rust-gnome-gtk,daa84/gtk,EPashkin/rust-gnome-gtk,EPashkin/rust-gnome-gtk,gkoz/rust-gnome-gtk,rust-gnome/gtk,Susurrus/gtk,rust-gnome/gtk