docs: Add README sample code (#34)

* fix: Correct link directive

* fix: Use a raw loop

Rob Pike would be proud

* fix: Correct flag usage

* docs: Add README sample code

Author: johnchildren

README.md

@@ -2,6 +2,39 @@
 
 "medium" level bindings to qulacs cppsim using cxxbridge.
 
+## Usage
+
+This library requires that QULACAS_PATH is set and points at the direction of a qulacs install.
+
+Additionally you will need to include a `build.rs` in your project that provides the -fopenmp
+flag to your linker.
+
+```rust
+fn main() {
+    println!("cargo:rustc-link-arg=-fopenmp");
+}
+```
+
+## Sample Code
+
+```rust
+let state = new_quantum_state(3, false);
+set_haar_random_state(&state);
+// Not a random state here.
+
+let circuit = new_quantum_circuit(3);
+add_x_gate(&circuit, 0);
+let merged_gate = merge(&new_cnot_gate(0, 1), &new_y_gate(1));
+add_gate_copy(&circuit, &merged_gate);
+add_r_x_gate(&circuit, 1, 0.5);
+update_quantum_state(&circuit, &state, 1);
+
+let observable = new_observable(3);
+add_operator(&observable, 2.0.into(), "X 2 Y 1 Z 0");
+add_operator(&observable, (-3.0).into(), "Z 2");
+let value = get_expectation_value(&observable, &state);
+```
+
 ## Development
 
 The recommended way to develop is using [devenv.sh](https://devenv.sh/) and [direnv](https://direnv.net/).

devenv.nix

@@ -1,18 +1,19 @@
 { pkgs, lib, config, inputs, ... }:
 let
-  qulacs = pkgs.callPackage ./nix/qulacs.nix {};
-in {
+  qulacs = pkgs.callPackage ./nix/qulacs.nix { };
+in
+{
   env = {
     QULACS_PATH = qulacs;
     LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
   };
-  
+
   packages = [
     pkgs.boost
     pkgs.eigen
     pkgs.mpi
     qulacs
-    
+
     pkgs.release-plz
     pkgs.cargo-semver-checks
   ];