README: try to clarify purpose of library

> Unfortunately that's not the scope or purpose of this library.

That actually clarifies things quite a bit for me. Wandering along from Hacker News, the README didn't really get that point across, that this is really intended as an embedded GUI kit for OpenGL/D3D sorts of applications. The screenshots and sample C++ code in the README don't show that context, either. When I went back and read it earlier today I caught both the "renderer agnostic" and the mentions in the Credits section. So I think we're getting on the same page here.

(I still have uses in mind, but they're very slightly different from what I was originally thinking.)