Add docs for CSV export

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What?

Adds documentation for CSV export of issues

Preview here: https://gitlab.com/gitlab-org/gitlab-ee/blob/jej-add-docs-for-csv-export/doc/user/project/issues/csv_export.md

Anything to review?

Feel free to use the web editor to make changes, I've marked this as "Squash commits when MR is accepted".