Add bugzilla to list because people ask about it.

doc/integration/external-issue-tracker.md

 # External issue tracker
 
-GitLab has a great issue tracker but you can also use an external issue tracker such as JIRA or Redmine. This is something that you can turn on per GitLab project. If for example you configure JIRA it provides the following functionality:
+GitLab has a great issue tracker but you can also use an external issue tracker such as JIRA, Bugzilla or Redmine. This is something that you can turn on per GitLab project. If for example you configure JIRA it provides the following functionality:
 
 - the 'Issues' link on the GitLab project pages takes you to the appropriate JIRA issue index;
 - clicking 'New issue' on the project dashboard creates a new JIRA issue;