Provide setup instructions

[AiyionPrime]

As we wasted some time on figuring out this packages relation with pythons isolation features and otherwise common pip install -e . pattern.

[TimSchimansky]

This finally helped!

[0xMihir]

I had to add <cstdint> to rasterizer_impl.h as mentioned here:
https://github.com/graphdeco-inria/diff-gaussian-rasterization/pull/49/files

[AiyionPrime]

@0xMihir That would be to have gcc13 compatibility, right?
I think it's fine for it to stay in the seperate PR :)

Eventually somebody from @graphdeco-inria or possibly @Snosixtyboo might review some of these PRs :)

But thanks for linking #49 as a heads-up for other researchers :)