Update youtube readme (langgenius#87)

Author: HanqingZ

tools/youtube/README.md

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+# YouTube
+
+## Overview
+
+[YouTube](https://www.youtube.com/) is the biggest online video sharing platform. Currently, Dify.ai has two relevant tools: `Video Statistics` to analyse the videos' information by entering the URL or keyword.
+
+## Configuration
+
+### 1. Apply for YouTube API Key
+
+Ensure that you are allowed to use Google Cloud Service
+
+> If you don't have an account, go to the [Google credential site](https://console.cloud.google.com/apis/credentials) and follow their instructions to create an account.
+
+If you have the account, go to the APIs & Services page and click `Create credentials -> API key` to create an API key.
+
+![](./_assets/youtube_1.PNG)
+
+Follow the step and click `Enabled APIs and services -> YouTube Data API v3` to enable the YouTube Data API.
+
+### 2. Setup the YouTube API in the Dify Tool page
+
+On the Dify navigation page, click `Tools > Youtube > To Authorize` to fill in the API Key.
+
+![](./_assets/youtube_2.PNG)
+
+### 3. Using the tool
+
+You can use the YouTube tool in the following application types:
+
+![](./_assets/youtube_3.PNG)
+
+#### Chatflow / Workflow applications
+
+Both Chatflow and Workflow applications support adding **Video statistics** nodes. 
+
+#### Agent applications
+
+Add the **Video statistics** tool in the Agent application, then enter relevant commands to invoke this tool.