Update README: How to use

README.md

@@ -6,3 +6,34 @@ Type definitions and simple functions inspired by Rust's Result.
 ```console
 npm i -D @yajamon/result.ts
 ```
+
+## Usage
+
+```ts
+import { Result, Ok, Err } from "@yajamon/result.ts"
+
+class Foo {
+  static create(input:string): Result<Foo, Error> {
+    if (validation(input)) {
+      const foo = new Foo();
+      // ...
+      return Ok(foo);
+    } else {
+      return Err(new Error("error message"));
+    }
+  }
+
+  // ...
+}
+
+function bar(input:string) {
+  let foo = Foo.create(input);
+  if (foo.isError === true) {
+    // foo is ResultErr<Error>
+    console.log(foo.error);
+    return;
+  }
+  // foo is ResultOk<Foo>
+  console.log(foo.value);
+}
+```