Add a shell.nix and instructions for using it

[ceedubs]

I have mixed feelings about this. On the one hand, it feels like which
tools a developer uses in their environment shouldn't bleed into the
source repository. On the other hand, getting a proper Cats development
environment includes wrangling some finicky bits that can be tricky for
a Scala developer (such as managing Jekyll and Node.js versions). I'm
generally in favor of lowering this barrier and using a deterministic
build environment.

I've been using something like this for a while when working on Cats and
thought that I'd put together this PR in case others are interested in
doing the same. If people aren't crazy about this, it's no big deal;
it's easy enough to have it ignored by git.

[ceedubs]

@tpolecat I've seen you comment on the mixed reviews that you've seen of Nix. For what it's worth, if you are on a Mac, I don't think that Nix is quite there as a Homebrew replacement, but I think that it works well for isolated development environments such as the one described in this PR. (I've also been using it as a Homebrew replacement, but it has required a lot of tinkering and some looking at source code, and not everything that I would want is there -- I think that the story is probably a bit better if you are running NixOS).

[codecov-io]

Codecov Report

Merging #2654 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2654      +/-   ##
==========================================
- Coverage   95.12%   94.86%   -0.26%     
==========================================
  Files         363      363              
  Lines        6704     6684      -20     
  Branches      289      283       -6     
==========================================
- Hits         6377     6341      -36     
- Misses        327      343      +16

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <0%> (-36.67%)	⬇️
core/src/main/scala/cats/syntax/either.scala	86.76% <0%> (-12.5%)	⬇️
.../src/main/scala/cats/data/RepresentableStore.scala	83.33% <0%> (-4.17%)	⬇️
core/src/main/scala/cats/data/IorT.scala	97.74% <0%> (-0.05%)	⬇️
core/src/main/scala/cats/Eval.scala	98.78% <0%> (-0.05%)	⬇️
core/src/main/scala/cats/data/Ior.scala	98.52% <0%> (-0.02%)	⬇️
core/src/main/scala/cats/data/ContT.scala	100% <0%> (ø)	⬆️
laws/src/main/scala/cats/laws/MonadLaws.scala	100% <0%> (ø)	⬆️

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[ceedubs]

This code coverage report makes no sense to me...

[rossabaker]

I think this is harmless for those who don't know Nix it and useful for those who do, or are willing to try.

[kailuowang]

👍

[LukaJCB]

I haven't used Nix much yet (though I definitely want to), but I think this is small enough to be perfectly fine as part of this Repo :)