docs: Add Twilio SMS plugin documentation (#1405)

docs/content/add-plugins/twilio-sms.md

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+# Twilio SMS
+
+In this document, you’ll learn about the Twilio SMS Plugin, what it does, and how to use it in Medusa.
+
+## Overview
+
+[Twilio’s SMS API](https://www.twilio.com/sms) can be used to send users SMS messages instantly. It has a lot of additional features such as Whatsapp messaging and conversations.
+
+By integrating Twilio SMS into Medusa, you’ll have easy access to Twilio’s SMS API to send SMS messages to your users and customers. You can use it to send Order confirmations, verification codes, reset password messages, and more.
+
+:::note
+
+This plugin only gives you access to the Twilio SMS API but does not implement sending messages at any given point. You’ll have to add this yourself where you need it. You can look at the [example later in this tutorial](#example-usage-of-the-plugin) to check how you can send an SMS for a new order.
+
+:::
+
+## Prerequisites
+
+Before going further with this guide make sure you have a Medusa server set up. You can follow our [Quickstart guide](../quickstart/quick-start.md) if you don’t.
+
+You also must have a [Twilio account created](https://www.twilio.com/sms) so if you don’t already please go ahead and create one.
+
+## Retrieve Credentials
+
+For the [Twilio SMS plugin](https://github.com/medusajs/medusa/tree/master/packages/medusa-plugin-twilio-sms), you need three credentials from your Twilio account: Account SID, Auth Token, and a Twilio phone number to send from. You can find these 3 from your [Twilio Console’s homepage](https://console.twilio.com).
+
+## Install Plugin
+
+In the directory of your Medusa server, run the following command to install [Twilio SMS plugin](https://github.com/medusajs/medusa/tree/master/packages/medusa-plugin-twilio-sms):
+
+```bash npm2yarn
+npm install medusa-plugin-twilio-sms
+```
+
+Then, you’ll need to add your credentials in `.env`:
+
+```bash
+TWILIO_SMS_ACCOUNT_SID=<YOUR_ACCOUNT_SID>
+TWILIO_SMS_AUTH_TOKEN=<YOUR_AUTH_TOKEN>
+TWILIO_SMS_FROM_NUMBER=<YOUR_TWILIO_NUMBER>
+```
+
+Make sure to replace `<YOUR_ACCOUNT_SID>`, `<YOUR_AUTH_TOKEN>`, and `<YOUR_TWILIO_NUMBER>` with the credentials you obtained from your Twilio Console.
+
+Finally, add the plugin and its options in the `medusa-config.js` file to the `plugins` array:
+
+```jsx
+const plugins = [
+  ...,
+  {
+    resolve: `medusa-plugin-twilio-sms`,
+    options: {
+      account_sid: process.env.TWILIO_SMS_ACCOUNT_SID,
+      auth_token: process.env.TWILIO_SMS_AUTH_TOKEN,
+      from_number: process.env.TWILIO_SMS_FROM_NUMBER
+    }
+  }
+];
+```
+
+## Example Usage of the Plugin
+
+This plugin adds the service `twilioSmsService` to your Medusa server. To send SMS using it, all you have to do is resolve it in your file as explained in the [Services](../advanced/backend/services/create-service.md#using-your-custom-service) documentation.
+
+In this example, you’ll create a subscriber that listens to the `order.placed` event and sends an SMS to the customer to confirm their order.
+
+:::tip
+
+For this example to work, you’ll need to install and configure Redis on your server. You can refer to the [development guide](../tutorial/0-set-up-your-development-environment.md#redis) to learn how to do that.
+
+:::
+
+Create the file `src/services/sms.js` in your Medusa server with the following content:
+
+```jsx
+class SmsSubscriber {
+  constructor({ twilioSmsService, orderService, eventBusService }) {
+    this.twilioSmsService_ = twilioSmsService;
+    this.orderService = orderService;
+
+    eventBusService.subscribe("order.placed", this.sendSMS);
+  }
+
+  sendSMS = async (data) => {
+    const order = await this.orderService.retrieve(data.id, {
+      relations: ['shipping_address']
+    });
+
+    if (order.shipping_address.phone) {
+      this.twilioSmsService_.sendSms({
+        to: order.shipping_address.phone,
+        body: 'We have received your order #' + data.id,
+      })
+    }
+  };
+}
+
+export default SmsSubscriber;
+```
+
+In the `constructor`, you resolve the `twilioSmsService` and `orderService` using dependency injection to use it later in the `sendSMS` method.
+
+You also subscribe to the event `order.placed` and sets the event handler to be `sendSMS`.
+
+In `sendSMS`, you first retrieve the order with its relation to `shipping_address` which contains a `phone` field. If the phone is set, you send an SMS to the customer using the method `sendSms` in the `twilioSmsService`.
+
+This method accepts an object of parameters. These parameters are based on Twilio’s SMS APIs. You can check their [API documentation](https://www.twilio.com/docs/sms/api/message-resource#create-a-message-resource) for more fields that you can add.
+
+If you create an order now on your storefront, you should receive a message from Twilio on the phone number you entered in the shipping address.
+
+:::tip
+
+If you don’t have a storefront set up yet, you can install one of our [Next.js](../starters/nextjs-medusa-starter.md) or [Gatsby](../starters/gatsby-medusa-starter.md) storefronts.
+
+:::
+
+:::caution
+
+If you’re on a Twilio trial make sure that the phone number you entered on checkout is a [verified Twilio number on your console](https://console.twilio.com/us1/develop/phone-numbers/manage/verified).
+
+:::
+
+![Twilio Dashboard](https://i.imgur.com/MXtQMiL.png)
+
+## What’s Next 🚀
+
+- Learn more about how [Notifications work in Medusa](../how-to/notification-api).
+- Install the [Medusa admin](../admin/quickstart.md) for functionalities like Gift Cards creation, swaps, claims, order return requests, and more.