Add howto for git modules

I finally found a "solution" for the git submodule issue (posted at https://stackoverflow.com/questions/66773544/git-submodule-not-ignored-even-with-ignore-all/79176853#79176853). -- Background: I was about to file a bug against lazygit, but then I found out that git add -A ignores the ignore = all setting. After some googling, I found the solution to use git update-index --assume-unchanged {sub-module-path}.

This PR adds this to the documetation.

[x] I own the copyright of the code submitted and I licence it under the MIT license
[ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
[ ] Tests created for changes (if applicable)
[ ] Manually tested changed features in running JabRef (always required)
[ ] Screenshots added in PR description (for UI changes)
[ ] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
[ ] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

JabRef / jabref