adding a README for the web gesture recognizer demo

examples/gesture_recognizer/js/README.md

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+# MediaPipe GestureRecognizer task for web
+
+## Overview
+
+This web sample detects hand landmarks in images and video frames and classifies gestures (gesture name and confidence level). Gestures are detected in one of the provided images or from camera frames seen by your device's webcam.
+
+## Prerequisites
+
+* A device that can access the web using Chrome, Firefox, or Safari
+* For iOS devices, iOS 16 or later
+
+## Running the demo
+
+The live demo and code are available in Codepen.
+
+[View the example on Codepen](https://codepen.io/mediapipe-preview/pen/zYamdVd)