Proposal: Nix development environment and package

[jpotier]

(I'm opening this MR mostly as a discussion, and although I'd like to see some of these changes pulled upstream, it is in no way blocking anyone from working with the current project as it is. No pressure)

Adding two files:

• shell.nix provides a shell environment for development. In the project
  directory, entering the development shell by invoking nix-shell will pull
  all required dependencies to build the project (based on the latest stable
  release).
• default.nix provides a package description of the latest stable release of
  the project. This allows people to quickly install m8c on their machine, by
  referencing the github project.

Added some documentation on how to use nix to automatically pull
dependencies, or quickly install m8c locally.

[laamaa]

Thank you, the package files are a good addition. Could you please describe how to maintain them on updated versions? Just update the version and hash in default.nix?

It might be better to leave only the bonus stuff in the readme. The installation part is meant to be more of a newbie guide, the part in the middle might just confuse most people just trying to get the thing running on their Raspberry Pi/whatever. I would imagine that Nix/Nixos users already know their way around the system :)

[jpotier]

I'll add a mention on how to maintain the files (as a comment maybe, or in a CONTRIBUTING.md) when I can (probably in the evening today)

[jpotier]

@laamaa I've added some docs, and removed all but the bonus stuff in README, is that better?

btw, the nix support for building and dev env should work in OSX as-is (untested), but also in Windows WSL2 (untested).

[laamaa]

Looks good to me, merged