Update the README with a better description of conrod

[mitchmindtree]

For the most part I think the existing description is OK, however conrod has very much grown into more of an "immediate mode GUI toolkit", rather than an easy-to-use, out of the box experience. E.g. while it is not as easy to use out-of-the-box as azul or druid, conrod is unique in its ability to be driven by any event loop (e.g. winit, SDL, glfw, gtk) and support custom renderers allowing a wide range of backends (e.g. wgpu, rendy, vulkano, glium, old gfx, piston). It is perfectly possible to make a very high-level, easy-to-use, out-of-the-box experience on top of conrod (as demonstrated in the nannou ui examples), however the goal of doing so is out of scope for conrod itself.