Document nix as an installation option?

[rasmus-kirk]

Flix is in nixpkgs, so I suggest adding nix as an installation option to the getting started page. Something like:

## Using nix

Flix can also be installed using the nix package manager. To install for the currently running shell run:

`nix-shell -p flix`

Or alternatively to install globally:

`nix-env -i flix`

Then run `flix run` in your project directory.

I can add a PR if you wish that this be added.

[magnus-madsen]

A PR would be welcome 👍

Please put it after using Flix from the command line. Thanks!

[magnus-madsen]

Fixed in master.