add a shell.nix

[evils]

this makes it a lot easier to set up an environment that has the dependencies
(with Nix installed, one only has to invoke nix-shell and you can run python3 main.py)

i had a bunch more dependencies in this, but removing them seems to make no difference to the startup info
i assume the kivy package supplies most of the ones that are actually needed at runtime

setuptools
gst-python
pillow
SDL2
pkg-config
libGL
gst_all_1.gstreamer
git
mtdev
xsel

i don't have a smoothieboard at hand yet to confirm any functionality beyond starting smoopi though

[evils]

tested this, seems to work

i also added a shebang to, and made main.py executable
so now usage is as simple as nix-shell --run './main.py'