[web] Added instructions on disabling git lfs.

web/content/docs/devguide/advanced/gitlab-migration.pandoc

@@ -9,6 +9,26 @@ weight = 1043
     parent = "advanced"
 +++
 
+To migrate your repository run the following commands in your local repository (`cd path/to/ogs`):
+
+## Disable Git LFS
+
+We used Git LFS to store large files but [switched back to plain git](https://github.com/ufz/ogs/issues/2961). You need to disable Git LFS in your repo:
+
+```bash
+git lfs uninstall --local
+```
+
+::: {.note}
+### <i class="far fa-info-circle"></i> Normalize LFS files in your existing branches
+If you have branches from pre-GitLab times with e.g. newly created LFS files (benchmark files, images, ...) you have to convert them back to plain git:
+
+```bash
+git add --renormalize .
+git commit -m "Converted LFS files to plain git."
+```
+:::
+
 ## Migrate your local repos to point to GitLab
 
 You have to modify your git remotes to point to the new GitLab repos. Assuming the former official git repo remote is called `upstream`: