Add disk space requirements to code quality doc

- CodeQuality reports can be quite large if your ruleset and project are
  both large. This causes issues when users pipelines fail and it is
difficult to tell it might be because of running out of disk space.

doc/user/project/merge_requests/code_quality.md

@@ -53,8 +53,10 @@ also requires the GitLab Runner 11.5 or later. For earlier versions, use the
 
 This example shows how to run Code Quality on your code by using GitLab CI/CD and Docker.
 
-First, you need GitLab Runner with
-[docker-in-docker executor](../../../ci/docker/using_docker_build.md#use-docker-in-docker-workflow-with-docker-executor).
+First, you need GitLab Runner with:
+
+- The [docker-in-docker executor](../../../ci/docker/using_docker_build.md#use-docker-in-docker-workflow-with-docker-executor).
+- Enough disk space to handle generated Code Quality files. For example on the [GitLab project](https://gitlab.com/gitlab-org/gitlab) the files are approximately 7 GB.
 
 Once you set up the Runner, include the CodeQuality template in your CI config: