added slack documentation (#1248)

docs/content/add-plugins/slack.md

@@ -1,3 +1,101 @@
-# Slack (Documentation coming soon)
+# Slack
 
-[View plugin here](https://github.com/medusajs/medusa/tree/master/packages/medusa-plugin-slack-notification)
+In this documentation, you'll learn how to add the [Slack plugin](https://github.com/medusajs/medusa/tree/master/packages/medusa-plugin-slack-notification) to your Medusa server to start receiving order notifications.
+
+## Overview
+
+When you add this plugin, the store owner can receive order notifications into their Slack workspace.
+
+The notification contains details about the order including:
+
+- Customer's details and address
+- Items ordered, their quantity, and the price
+- Order totals including Tax amount.
+- Promotion details if there are any (this is optional and can be turned off).
+
+The plugin registers a subscriber to the `order.placed` event. When an order is placed, the subscriber handler method uses the ID of the order to retrieve order details mentioned above.
+
+Then, the order notificaiton is sent to Slack using Webhooks. So, you'll need to create a Slack App, add it into your workspace, and activate Incoming Webhooks.
+
+## Prerequisites
+
+### Slack Account
+
+To follow along with this guide, you need to have a Slack account with a connected workspace. If you don’t have one, you can [create a free account on Slack](https://slack.com/).
+
+### Medusa Server
+
+This tutorial assumes you already have a Medusa server installed. If you don’t, please follow along with the [quickstart guide](../quickstart/quick-start.md).
+
+### Redis
+
+Medusa's event system works by pushing data into a queue that is based on [Redis](https://redis.io/). This queue then notifies handlers of different events of this data that is pushed into the queue. The handlers then use this data to perform a certain action.
+
+As the Slack plugin will listen to the `order.placed` event to know when to send notifications, you'll need to have Redis installed and configured with your Medusa server.
+
+You can read the [Set up your development enviornment guideline](https://docs.medusajs.com/tutorial/set-up-your-development-environment) to learn more about how you can install and setup Redis.
+
+## Create Slack App
+
+The first step is to create a Slack app. This app will be connected to your workspace and will have Incoming Webhooks activated to receive notifications from the Medusa server using a Webhook URL.
+
+Go to [Slack API](https://api.slack.com/) and click Create app. This will take you to a new page with a pop-up. In the pop-up, choose From scratch.
+
+![Create Slack App](https://i.imgur.com/liVfwF8.png)
+
+You’ll then need to enter some info like the App name and the workspace it will be connected to. Once you’re done, the app will be created.
+
+### Activate Incoming Webhooks
+
+To activate Incoming Webhooks, choose Features > Incoming Webhooks from the sidebar. At first, it will be disabled so make sure to enable it by switching the toggle.
+
+![Incoming Webhooks](https://i.imgur.com/5Y0nv4p.png)
+
+### Add New Webhook
+
+After activating Incoming Webhooks, on the same page scroll down and click on the Add New Webhook to Workspace button.
+
+![Add New Webhook](https://i.imgur.com/sejdIqH.png)
+
+After that, choose the channel to send the notifications to. You can also choose a DM to send the notifications to. Once you’re done click Allow.
+
+![Choose channel or DM](https://i.imgur.com/Zw3f5uF.png)
+
+This will create a new Webhook with a URL which you can see in the table at the end of the Incoming Webhooks page. Copy the URL as you’ll use it in the next section.
+
+## Install Slack Plugin
+
+The next step is to install Medusa’s [Slack plugin](https://github.com/medusajs/medusa/tree/master/packages/medusa-plugin-slack-notification) into your Medusa server.
+
+Open the terminal in the Medusa server’s directory and run the following command:
+
+```bash
+npm install medusa-plugin-slack-notification
+```
+
+After that, open `medusa-config.js` and add the new plugin with its configurations in the `plugins` array:
+
+```jsx
+const plugins = [
+  ...,
+  {
+    resolve: `medusa-plugin-slack-notification`,
+    options: {
+      show_discount_code: false,
+      slack_url: `<WEBHOOK_URL>`,
+      admin_orders_url: `http://localhost:7001/a/orders`
+    }
+  }
+];
+```
+
+- Make sure to change `<WEBHOOK_URL>` with the Webhook URL you copied after creating the Slack app.
+- The `show_discount_code` option enables or disables showing the discount code in the notification sent to Slack. 
+- The `admin_orders_url` is the prefix of the URL of the order detail pages on your admin panel. If you’re using Medusa’s Admin locally, it should be `http://localhost:7001/a/orders`. This will result in a URL like `http://localhost:7001/a/orders/order_01FYP7DM7PS43H9VQ1PK59ZR5G`.
+
+That’s all you need to do to integrate Slack into Medusa!
+
+## What's Next :rocket:
+
+- Install [Medusa's Admin](https://github.com/medusajs/admin) for the full order-management experience.
+- Add a Storefront to your Medusa server using [the Next.js starter](https://docs.medusajs.com/starters/nextjs-medusa-starter) or [the Gatsby starter](https://docs.medusajs.com/starters/gatsby-medusa-starter).