docs: write docs

docs/usage.md

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+# Usage
+
+## Basic Shell
+
+The absolute minimum to get started:
+
+```nix
+devshell.mkShell {}
+```
+
+That's it. You've got yourself a shell. Not very exciting, but it works.
+
+## Adding Packages
+
+Want some tools? Throw them in:
+
+```nix
+devshell.mkShell {
+  packages = [ pkgs.hello pkgs.git pkgs.nodejs ];
+}
+```
+
+## Environment Variables
+
+Set some env vars (because who doesn't love those):
+
+```nix
+devshell.mkShell {
+  env = {
+    HELLO.value = "world";
+  };
+}
+```
+
+## Shell Hooks
+
+Run stuff when entering the shell:
+
+```nix
+devshell.mkShell {
+  enterShellCommands."hello".text = ''
+    echo "Welcome to the devshell!"
+    echo "Current directory: $PWD"
+  '';
+}
+```
+
+## Putting It All Together
+
+A more realistic example:
+
+```nix
+devshell.mkShell {
+  packages = [ pkgs.nodejs pkgs.yarn pkgs.git ];
+  
+  env = {
+    NODE_ENV.value = "development";
+    API_URL.value = "http://localhost:3000";
+  };
+  
+  enterShellCommands."enter".text = ''
+    echo "🚀 Development environment ready!"
+    node --version
+  '';
+}
+```
+
+## Need More Features?
+
+For additional functionality, import modules:
+
+```nix
+devshell.mkShell {
+  imports = [ some-module.devshellModule ];
+  # your config...
+}
+```
+
+Check out [available modules](./integrations.md) for things like automatic file
+generation, dev tools and more.