Add dev documentation for using the QueryRecorder in specs

A tool for preventing N+1 regressions was added in https://gitlab.com/gitlab-org/gitlab-ce/commit/9c623e3e5d7434f2e30f7c389d13e5af4ede770a and used in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/features/issuables/issuable_list_spec.rb#L16-22

I'll add this to the performance documentation