update readme

Author: kartikkhk

README.md

@@ -1,2 +1,26 @@
 # queue-manager
+
 A wrapper over [async](https://caolan.github.io/async/v3/docs.html) queue helpful in managing the queue length
+
+### Usage example
+
+```javascript
+// inputArray of huge length (approx 1000 or more)
+const lowWaterMark = 40; // whatever you want it to be
+const highWaterMark = 100; // whatever you want it to be
+
+function main(inputArray) {
+  const queue = async.queue(worker, 20);
+  async function worker(input, cb) {
+    console.log("queue length -->", queue.length());
+    if (queue.length() <= lowWaterMark) queueManager.resume();
+    await doSomething(input);
+    return cb();
+  }
+  const queueManager = new QueueManager(queue);
+  for (const input of inputArray) {
+    if (queue.length() >= highWaterMark) await queueManager.pause(); // pauses the loop till queue length has decreased below low watermark
+    queue.push(input);
+  }
+}
+```