Add instructions on setting up GitLab SSH to README

After moving to use SSH remote URLs in the requirements Karim pointed out that it's possible that detector scientists and others may not have SSH keys set up yet, so it'd be useful to have a small section on this in the readme.

Description

Adds a section to the appendix describing how to set up SSH keys for EuXFEL GitLab access.

How Has This Been Tested?

N/A

Relevant Documents (optional)

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Reviewers

@ahmedk @danilevc @hammerd