docs: documentation for slack integration (runatlantis#1972)

* docs: add docs for slack integration

* add menu

runatlantis.io/.vuepress/config.js

@@ -72,7 +72,8 @@ module.exports = {
                         'apply-requirements',
                         'checkout-strategy',
                         'terraform-versions',
-                        'terraform-cloud'
+                        'terraform-cloud',
+                        'using-slack-hooks'
                     ]
                 },
                 {

runatlantis.io/docs/using-slack-hooks.md

@@ -0,0 +1,46 @@
+# Using Slack hooks
+
+It is possible to use Slack to send notifications to your Slack channel whenever an apply is being done.
+
+::: tip NOTE
+Currently only `apply` events are supported.
+:::
+
+For this you'll need to:
+
+* Create a Bot user in Slack
+* Configure Atlantis to send notifications to Slack.
+
+## Configuring Slack for Atlantis
+
+* Go to [https://api.slack.com/apps](https://api.slack.com/apps)
+* Click the `Create New App` button
+* Select `From scratch` in the dialog that opens
+* Give it a name, e.g. `atlantis-bot`.
+* Select your Slack workspace
+* Click `Create App`
+* On the left go to `oAuth & Permissions`
+* Copy the `Bot User OAuth Token` and provide it to Atlantis by using `--slack-token=xoxb-xxxxxxxxxxx` or via the environment `ATLANTIS_SLACK_TOKEN=xoxb-xxxxxxxxxxx`.
+* Scroll down to scopes and add the following:
+  * `channels:read`
+  * `chat:write`
+  * `groups:read`
+  * `incoming-webhook`
+  * `mpim:read`
+* Install the app onto your Slack workspace
+
+## Configuring Atlantis
+
+After following the above steps it is time to configure Atlantis. Assuming you have already provided the `slack-token` (via parameter or environment variable) you can now instruct Atlantis to send `apply` events to Slack.
+
+In your Atlantis configuration you can now add the following:
+
+```yaml
+webhooks:
+- event: apply
+  workspace-regex: .*
+  kind: slack
+  channel: my-channel
+```
+
+The `apply` event information will be sent to the `my-channel` Slack channel.