Add instructions on how to use bend with nix

[k3yss]

Bend recently got packaged for Nix ( link ) . This PR adds instructions to README.md on how to use Bend with Nix.

[developedby]

We're writing more extensive documentation at the moment. We'll put this in a more in-depth install section with instructions for other methods like nix

[Fidanze]

@k3yss , does this nix-package support cuda? i have cudaToolkit and bend run-cu keep give me CUDA runtime not available! If you've installed CUDA and nvcc after HVM, please reinstall HVM. Errors: Failed to parse result from HVM. is it normal?