Outline developer docs for setting up Group SAML

What

We need to describe how to get a test identity provider running and might need to explain how to setup GitLab on https to work in conjunction with this.

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5817#note_75805181 and gitlab-qa#257 (comment 75817148)

Why

So developers reviewing, fixing or extending GitLab.com Group SAML can easily get something to work with.