Document uploads development guidelines
This MR aims to translate the discussion on https://gitlab.com/gitlab-org/gitlab-ce/issues/63097 into a development guide.
It will be easier to discuss in the form of a MR.
http://review-docs-ac-up-x7z9bn.178.62.207.141.xip.io/ce/development/uploads.html
Edited by Alessio Caiazza