Add ticket reference to Depfu setup (#422)

* Add ticket reference to Depfu setup

In khw we use ticket references in our Depfu configuration:
renuo/khw2#606

I propose that we set the reference by default.

* Apply suggestion from @CuddlyBunion341

* Update image link

Author: cb341

images/depfu_pr_title.png

@@ -5,7 +5,7 @@ dependencies. It will automatically create a pull request to the project
 if a security vulnerability has been disclosed.
 
 1. Ask wg-operations to add repository access for Depfu to you new Github
-repository.
+   repository.
 
 That's all :-)
 
@@ -21,8 +21,8 @@ Enable minor engine updates.
 be available on their platform, so you may need to delay the upgrade. Check the
 following GitHub repositories to see if Heroku added support already:
 
-* <https://github.com/heroku/heroku-buildpack-nodejs>
-* <https://github.com/heroku/heroku-buildpack-ruby>
+- <https://github.com/heroku/heroku-buildpack-nodejs>
+- <https://github.com/heroku/heroku-buildpack-ruby>
 
 ## Automatic merging
 
@@ -55,3 +55,12 @@ jobs:
           PR_URL: ${{github.event.pull_request.html_url}}
           GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
 ```
+
+## Ticket reference
+
+In order to avoid searching for the maintenance project after reviewing a Depfu PR,
+we add the ticket to the PR title according to the following Depfu configuration:
+
+![Depfu Redmine Ticket PR Title](../images/depfu_pr_title.png)
+
+Make sure that the ticket number matches the actual maintenance ticket.