After running conda jobs on tags, the following failed:
https://gitlab.astro-wise.org/omegacen/common/-/pipelines/10844
This was due to the tags created by the MM job not being protected,
but the CONDA_UPLOAD_KEY (set globally by an admin) was.

This has been fixed for the common project: the tags `20*` are now
protected.

This MR adds a bit of documentation to other confused project owners
can understand what's going on.