Starter guide: Working with git

[sstroemer]

As raised by @nyashags a low-entry-barrier guide on how to work with git (and our internal GitLab) may be helpful for people that are just getting started working with IESopt / in a team.

A starting point can be found in Developer documentation > Contributing, but a separate page would be helpful. I kinda imagine the following structure

.
└── Developer documentation/
    ├── General/
    │   ├── Contributing
    │   └── Working with git
    ├── Python/
    │   ├── Documentation
    │   ├── Coding
    │   └── Testing
    └── Julia/
        ├── Documentation
        ├── Coding
        └── Testing

So (besides other open points) we are in need of the Working with git section.