Add `devShell` example (#31)

* Add `devShell` example

Co-authored-by: oxalica <oxalicc@pm.me>

README.md

@@ -50,6 +50,8 @@ $ cargo --version
 cargo 1.49.0 (d00d64df9 2020-12-05)
 ```
 
+### Example: NixOS Configuration
+
 Here's an example of using it in nixos configuration.
 ```nix
 {
@@ -77,6 +79,49 @@ Here's an example of using it in nixos configuration.
 }
 ```
 
+### Example: Using `devShell` and `nix develop`
+
+Running `nix develop` will create a shell with the default nightly Rust toolchain installed:
+
+```nix
+{
+  description = "A devShell example";
+
+  inputs = {
+    nixpkgs.url      = "github:nixos/nixpkgs/nixos-unstable";
+    rust-overlay.url = "github:oxalica/rust-overlay";
+    flake-utils.url  = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let 
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs {
+          inherit system overlays;
+        };
+      in
+      {
+        devShell = pkgs.mkShell {
+          buildInputs = [
+            pkgs.openssl
+            pkgs.pkgconfig
+            pkgs.exa
+            pkgs.fd
+            pkgs.rust-bin.nightly.latest.default
+          ];
+
+          shellHook = ''
+            alias ls=exa
+            alias find=fd
+          '';
+        };
+      }
+    );
+}
+
+```
+
 ## Attributes provided by the overlay
 
 ```nix