README.md: Add devenv to Command-Line tools (#205)

Add the devenv.sh tool to the list of awesome command-line tools.

Closes #204

Author: cyntheticfox

README.md

@@ -104,6 +104,7 @@
 * [alejandra](https://github.com/kamadorueda/alejandra) - An opinionated Nix code formatter optimized for speed and consistency.
 * [comma](https://github.com/nix-community/comma) - Quickly run any binary; wraps together `nix run` and `nix-index`.
 * [deadnix](https://github.com/astro/deadnix) - Scan Nix files for dead code.
+* [devenv](https://github.com/cachix/devenv) - A Nix-based tool for creating developer shell environments quickly and reproducibly.
 * [manix](https://github.com/mlvzk/manix) - Find configuration options and function documentation for Nixpkgs, NixOS, and Home Manager.
 * [nixfmt](https://github.com/serokell/nixfmt) - A formatter for Nix code, intended to easily apply a uniform style.
 * [nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt) - Nix code formatter for nixpkgs.