docs: improve README and add small improvement for usage

Minimal devshell implementation using Modules.

Inspired by [numtide/devshell](https://github.com/numtide/devshell).

## Features

- **fast**: due to the NixOS module system it's a bit slower than `pkgs.mkShell`, but only about 10% (still in the tens of milliseconds range)

- **minimal**: only contains the bare minimum, the rest should be added with imported modules

- **straight forward**: no toml (like numtide/devshell), no abstractions over Nix (like devenv), just a straight forward Nix lib

## Usage

Simple example:

```nix

let

  # make sure to add 

  #  inputs.devshell.url = "gitlab:rensa-nix/devshell?dir=lib";

  # to your inputs

  devshell = inputs.devshell.lib { inherit pkgs; };

in

  devshell.mkShell {}

```

## Docs

See [docs](https://devshell.rensa.projects.tf)

The absolute minimum to get started:

```nix

let

  # make sure to add 

  #  inputs.devshell.url = "gitlab:rensa-nix/devshell?dir=lib";

  # to your inputs

  devshell = inputs.devshell.lib { inherit pkgs; };

in

  devshell.mkShell {}

```