Register SignedDistanceFields.jl

Hey y'all,

I've been working on rendering text with OpenGL and have created a small package to compute signed distance fields. I'm planning on extending it with more functionality specific to computing SDFs for rendering with OpenGL, such as tolerance parameters and perhaps integration with FreeType to generate texture atlases, but I thought I'd submit what I have so far since it's already doing something useful.

Thanks!

Link to repository: https://github.com/yurivish/SignedDistanceFields.jl