document the need to be owner or have the master permission level for the initial push

## What does this MR do?

It documents the new behavior as discussed on gitlab-org/gitlab-ce#22543, because the `/protected_branch` URL of empty repository is not available and could mislead users.

## Are there points in the code the reviewer needs to double check?

Don't think so

## Why was this MR needed?

When you create a repository on GitLab, command line instructions on its home page for the use case _"Existing folder or Git repository"_ invite you to `git push -u origin master`, but the very first push is now rejected if you are not `Owner` or attributed `Master` permission.

## Screenshots (if relevant)

![Capture_d_écran_2016-09-30_à_14.55.20](/uploads/6ee2d36b4a32cb3a5faa68a10c1813f0/Capture_d_écran_2016-09-30_à_14.55.20.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

- new behavior was introduced here : gitlab-org/gitlab-ce@71dec8b1b61e9e194d242d37b39416b72020936b
- lack of documentation was discussed at gitlab-org/gitlab-ce#22543

See merge request !6608