Add link to a comparison to README

[MrRulf]

This Pull Request fixes/closes #{issue_num}.

It changes the following:

• It adds a link to an article comparing gitui to other tui git tools like lazygit. It's a bit outdated as it's from 2023 but it also gives a good idea what the tool can do.

I followed the checklist:

• [/] I added unittests
• [/] I ran make check without errors
• [/] I tested the overall application
• [/] I added an appropriate item to the changelog