Merge branch 'docs-trello-power-up' into 'master'

Add GitLab Trello Power-Up Documentation

See merge request !12997

doc/README.md

@@ -115,6 +115,7 @@ Manage files and branches from the UI (user interface):
 
 - [Project Services](user/project/integrations/project_services.md): Integrate a project with external services, such as CI and chat.
 - [GitLab Integration](integration/README.md): Integrate with multiple third-party services with GitLab to allow external issue trackers and external authentication.
+- [Trello Power-Up](integration/trello_power_up.md): Integrate with GitLab's Trello Power-Up
 
 ----
 

doc/integration/trello_power_up.md

+# Trello Power-Up
+
+GitLab's Trello Power-Up enables you to seamlessly attach
+GitLab **merge requests** to Trello cards.
+
+![GitLab Trello PowerUp - Trello card](img/trello_card_with_gitlab_powerup.png)
+
+## Configuring the Power-Up
+
+In order to get started, you will need to configure your Power-Up.
+
+In Trello:
+
+1. Go to your Trello board
+1. Select `Power-Ups` to see a listing of all the available Power-Ups
+1. Look for a row that says `GitLab` and select the `Enable` button
+1. Select the `Settings` (gear) icon
+1. In the popup menu, select `Authorize Account`
+
+In this popup, fill in your `API URL` and `Personal Access Token`. After that, you will be able to attach any merge request to any Trello card on your selected Trello board.
+
+## What is my API URL?
+
+Your API URL should be your GitLab instance URL with `/api/v4` appended in the end of the URL.
+For example, if your GitLab instance URL is `https://gitlab.com`, your API URL would be `https://gitlab.com/api/v4`.
+If your instance's URL is `https://example.com`, your API URL will be `https://example.com/api/v4`.
+
+![configure GitLab Trello PowerUp in Trello](img/enable_trello_powerup.png)
+
+## What is my Personal Access Token?
+
+Your GitLab's personal access token will enable your GitLab account to be accessed
+from Trello.
+
+> Find it in GitLab by clicking on your avatar (upright corner), from which you access
+your user **Settings** > **Access Tokens**.
+
+Learn more about generating a personal access token in the
+[Personal Access Token Documentation][personal-access-token-documentation].
+Don't forget to check the API scope checkbox!
+
+[personal-access-token-documentation]: ../user/profile/personal_access_tokens.html