add some useful debug/rendering commands to the readme

[Tectract]

These commands would be really useful for debugging some GPU issues with atom, they helped me. I recommend adding them to the readme:

atom --safe --disable-gpu
atom --safe --enable-gpu-rasterization --force-gpu-rasterization

In regards to GPU rendering and possible flicker debug issues on old GPUs, specifically ATI models on linux.

[lee-dohm]

I think this would make a good addition to the Debugging section. It would need some more explanation than just the parameters themselves. Care to take a stab at it?