Rework nix instructions now that nixpkgs has it

[thenonameguy]

now that this project is packaged in the main nixpkgs repostitory
the need for instructing people to compile from source is unnecessary.

I'm also proposing changes to include process-compose as a built-in component in this project:
https://devenv.sh/
cachix/devenv#78

The above would solve problems like #14
and provide nice common service descriptions for common architectural components (postgres/kafka/redis/etc.).

Might be worth mentioning in the README.md once the above issue is resolved.

[F1bonacc1]

Hey @thenonameguy,

I gave it a little more thought. You don't need to change your PR. I will initialize the string (it has to be string) flag to be "false", so if you pass nothing we will get the desired behavior.
I will update my makefile to pass "true", of course.