Add documentation.

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diff --git a/doc/README.md b/doc/README.md
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@@ -29,6 +29,7 @@
 - [System hooks](system_hooks/system_hooks.md) Notifications when users, projects and keys are changed.
 - [Update](update/README.md) Update guides to upgrade your installation.
 - [Welcome message](customization/welcome_message.md) Add a custom welcome message to the sign-in page.
+- [Reply by email](reply_by_email/README.md) Allow users to comment on issues and merge requests by replying to notification emails.
 
 ## Contributor documentation
 
diff --git a/doc/reply_by_email/README.md b/doc/reply_by_email/README.md
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+# Reply by email
+
+GitLab can be set up to allow users to comment on issues and merge requests by replying to notification emails.
+
+In order to do this, you need access to an IMAP-enabled email account, with a provider or server that supports [email sub-addressing](https://en.wikipedia.org/wiki/Email_address#Sub-addressing). Sub-addressing is a feature where any email to `user+some_arbitrary_tag@example.com` will end up in the mailbox for `user@example.com`, and is supported by providers such as Gmail, Yahoo! Mail, Outlook.com and iCloud, as well as the [Postfix](http://www.postfix.org/) mail server which you can run on-premises.
+
+## Set it up
+
+In this example, we'll use the Gmail address `gitlab-replies@gmail.com`. If you're actually using Gmail with Reply by email, make sure you have [IMAP access enabled](https://support.google.com/mail/troubleshooter/1668960?hl=en#ts=1665018) and [allow less secure apps to access the account](https://support.google.com/accounts/answer/6010255).
+
+### Installations from source
+
+
+1. Find the `reply_by_email` section in `config/gitlab.yml`, enable the feature and enter the email address including a placeholder for the `reply_key`:
+    
+    ```yaml
+    reply_by_email:
+      enabled: true
+      address: "gitlab-replies+%{reply_key}@gmail.com"
+    ```
+
+    As mentioned, the part after `+` is ignored, and this will end up in the mailbox for `gitlab-replies@gmail.com`.
+
+2. Find `config/mail_room.yml.example` and copy it to `config/mail_room.yml`:
+    
+    ```sh
+    cp config/mail_room.yml.example config/mail_room.yml
+    ```
+
+3. Uncomment the configuration options in `config/mail_room.yml` and fill in the details for your specific IMAP server and email account:
+
+    ```yaml
+    :mailboxes:
+      -
+        # IMAP server host
+        :host: "imap.gmail.com"
+        # IMAP server port
+        :port: 993
+        # Whether the IMAP server uses SSL
+        :ssl: true
+        # Email account username. Usually the full email address.
+        :email: "gitlab-replies@gmail.com"
+        # Email account password
+        :password: "[REDACTED]"
+        # The name of the mailbox where incoming mail will end up. Usually "inbox".
+        :name: "inbox"
+        # Always "sidekiq".
+        :delivery_method: sidekiq
+        :delivery_options:
+          # The URL to the Redis server used by Sidekiq. Should match the URL in config/resque.yml.
+          :redis_url: redis://localhost:6379
+          # Always "resque:gitlab".
+          :namespace: resque:gitlab
+          # Always "incoming_email".
+          :queue: incoming_email
+          # Always "EmailReceiverWorker"
+          :worker: EmailReceiverWorker
+    ```
+
+
+4.  Find `lib/support/init.d/gitlab.default.example` and copy it to `/etc/default/gitlab`:
+    
+    ```sh
+    sudo cp lib/support/init.d/gitlab.default.example /etc/default/gitlab
+    ```
+
+5. Edit `/etc/default/gitlab` to enable `mail_room`:
+
+    ```sh
+    mail_room_enabled=true
+    ```
+
+6. Restart GitLab
+    
+    ```sh
+    sudo service gitlab restart
+    ```
+
+8. Reply by email should now be working.
+
+Note: If you're running GitLab in development mode and using `foreman`, make sure to also uncomment the `mail_room` line in your `Procfile`.
+
+### Omnibus package installations
+
+TODO