Create how-to-use.md (#13972)

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+In this document, we will give a brief introduction on how to use the JavaScript SDK for new users
+
+1. Prepare your environment.  
+    NodeJS: can be installed from https://nodejs.org/en/download/  
+    typescript: install it with `npm install -g typescript`.
+
+1. Create a empty folder and cd this folder.
+    ```
+    mkdir jstest
+    cd jstest
+    ```
+
+1. Initialize a new node project. 
+    ```
+    npm init
+    ```
+    This step will create a package.json file in current folder.
+
+1. Install dependencies.
+   ```
+   // install the nodeauth packages to do the authentication work.
+   npm install @azure/ms-rest-nodeauth
+   // or install @azure/identity, if you would like to use @azure/identity to do the authentication work.
+   npm install @azure/identity
+   
+   // Then install your target try out package,  you can install the latest published with
+   npm install @azure/arm-XXX 
+   //  or install it from your local JS SDK artifact file. 
+   npm install D:\\dev\\test\\test-compute\\azure-arm-XXX-1.0.0.tgz
+   
+   ```
+   In the case of verifying the unpublished packages, you may download the artifact from either rest api specs CI pipeline or from the release request issue that we provided.  
+
+
+1. Create a ts file (free name and copy follow code into this file) eg: test_1.ts
+    eg：
+    ```
+        const msRestNodeAuth = require("@azure/ms-rest-nodeauth");
+        const { TargetManagementClient } = require("@azure/arm-target");  // must same as dependencies
+        const subscriptionId = process.env["AZURE_SUBSCRIPTION_ID"];
+
+        msRestNodeAuth.interactiveLogin().then((creds) => {
+            const client = new TargetManagementClient(creds, subscriptionId);
+            client.operations.list().then((result) => {      // you can test that you need test operation
+                console.log("The result is:");
+                console.log(result);
+            });
+        }).catch((err) => {
+                console.error(err);
+        });
+    ```   
+    A few points, you need to pay attention here:  
+    In the example, we are using msRestNodeAuth.interactiveLogin, you can also try other way such as Service Principal
+    ```
+        const credentials = await msRestNodeAuth.loginWithServicePrincipalSecret(clientID, clientSecret, tenantID);
+    ```
+    In the example, we only add client.operations.list(), you may change them into other resources CURD function per your need.  
+    for example：  
+    ```
+    const client = new ComputeManagementClient(credentials, subscriptionID);
+    await client.galleries.createOrUpdate(resourceGroupName, galleryName, gallery);
+    await client.galleryImages.createOrUpdate(resourceGroupName, galleryName, galleryImageName, galleryImage);
+    ```
+    
+1. Install all the dependencies 
+    ```
+    npm install // need to make sure package.json exists and has contained at step 4.
+    ```
+1. Compile the ts file.
+   ```
+   tsc test_1.ts
+   ```
+   it will generate a test_1.js file in current folder.
+1. Run the code. 
+   ```
+   node test_1.js
+   ```
+   Now you can see expected response.