Description of the problem, including code/CLI snippet

Since GitLab 11.5 there is an API for handling Git submodules.
https://docs.gitlab.com/ee/api/repository_submodules.html

I would love to help here, but i don't get how to add new API elements. python-gitlab looks like having a nice pluggable structure, but currently i do not understand what i need to do exactly. A short introduction for developers in the documentation would be nice.

Expected Behavior

Having submodules API available in python-gitlab

Actual Behavior

Submodules API not implemented

Specifications

• python-gitlab version: 1.11
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 12.2.4