doc: update readme

README.md

@@ -1,2 +1,21 @@
 # pipou
+
 Middleware builder
+
+## Usage
+
+Configure the middleware chain by calling `use()`.
+
+Then, `execute()` the chain: this will always return `Promise<void>`
+```js
+await new MiddlewareBuilder <{someValue: string}> ()
+    .use(async (message, next) => {
+        message.someValue = 'hi';
+        await next();
+    })
+    .use((message, _) => {
+        message.someValue = 'bonjour';
+    })
+    .execute({someValue: 'hello'});
+```
+