Update README.md

Co-authored-by: Thilo Fromm <[email protected]>

Author: John15321

README.md

@@ -104,6 +104,9 @@ To clone the scripts repo and pick a version:
 
 ### Working with forks
 
+When using GitHub's "fork" feature, please **make sure to fork all branches**, not just `main`. Forking only `main` is the default on GitHub,
+
+The SDK container wrapper script `run_sdk_container` requires release tags in our release branches and fails to start if no release branch is present (see e.g. https://github.com/flatcar/Flatcar/issues/1705).
 If you're working with a fork of the scripts repository, you'll need to fetch the upstream tags to avoid version detection issues:
 
 ```bash