Update documentation on JS API client (#25266)

* Update documentation on JS API client

* Update link to monorepo and web app repo

api/v4/source/introduction.yaml

@@ -57,7 +57,9 @@ tags:
 
       #### Official Drivers
 
-      * [Mattermost JavaScript Driver](https://github.com/mattermost/mattermost-redux/blob/master/src/client/client4.ts)
+      * Mattermost JavaScript/TypeScript Driver
+        * [NPM](https://www.npmjs.com/package/@mattermost/client)
+        * [Source](https://github.com/mattermost/mattermost/tree/master/webapp/platform/client)
 
       * [Mattermost Golang Driver](https://github.com/mattermost/mattermost/blob/master/server/public/model/client4.go)
 

webapp/platform/client/README.md

@@ -1,6 +1,64 @@
 # Mattermost Client
 
-This package contains the JavaScript/TypeScript client for [Mattermost](https://github.com/mattermost/mattermost-server). It's used by [the Mattermost web app](https://github.com/mattermost/mattermost-webapp) and related projects.
+[![npm version](https://img.shields.io/npm/v/@mattermost/client?style=flat)](https://www.npmjs.com/package/@mattermost/client)
+
+This package contains the JavaScript/TypeScript client for [Mattermost](https://github.com/mattermost/mattermost). It's used by [the Mattermost web app](https://github.com/mattermost/mattermost/tree/master/webapp/channels) and related projects.
+
+## Installation
+
+### JavaScript
+
+```sh
+$ npm install @mattermost/client
+```
+
+### TypeScript
+
+```sh
+$ npm install @mattermost/client @mattermost/types
+```
+
+## Usage
+
+To use the client, create an instance of `Client4`, set the server URL, and log in, and then you can start making requests.
+
+```js
+import {Client4} from '@mattermost/client';
+
+const client = new Client4();
+client4.setUrl('https://mymattermostserver.example.com');
+
+client4.login('username', 'password').then((user) => {
+    // ...
+});
+```
+
+If you already have a session token or a user access token, you can call `Client4.setToken` instead of logging in.
+
+```js
+import {Client4} from '@mattermost/client';
+
+const client = new Client4();
+client4.setUrl('https://mymattermostserver.example.com');
+
+client4.setToken('accesstoken');
+```
+
+If needed, methods exist to set other headers such as the User-Agent (`Client4.setUserAgent`), the CSRF token (`Client4.setCSRF`), or any extra headers you wish to include (`Client4.setHeader`).
+
+Methods of `Client4` which make requests to the server return a `Promise` which does the following:
+
+- On success, the promise resolves to a `ClientResponse<T>` object which contains the the [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) (`response`), a [Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) of headers (`headers`), and the data sent from the server (`data`).
+- On an error, the promise rejects with a `ClientError` which contains the error message and the URL being requested. If the error happened on the server, the status code and an error ID (`server_error_id`) are included.
+
+```js
+let user;
+try {
+    user = (await client.getUser('userid')).data;
+} catch (e) {
+    console.error(`An error occurred when making a request to ${e.url}: ${e.message}`);
+}
+```
 
 ## Compilation and Packaging