docs(nix): document nix develop usage for local testing

Add documentation for using 'nix develop' to test the Neovim
configuration locally before installation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -49,6 +49,10 @@ If you use Nix with flakes, you can install this configuration via the flake:
 # Try it out without installing
 nix run github:0xWheatyz/kickstart.nvim
 
+# Test locally with development shell (includes nvim + dependencies)
+nix develop
+nix develop -c nvim  # Or run nvim directly
+
 # Or add to your home-manager configuration
 {
   inputs.kickstart-nvim.url = "github:0xWheatyz/kickstart.nvim";