add howto card for mirroring fork

3 jobs for !225 with mirror-fork in 23 seconds (queued for 1 second)
merge request
Name Stage Failure
failed
save:commitIndex Save
$ git commit -m "Update index" || true
[tmpBranch b4fc6a5] Update index
1 file changed, 1 insertion(+)
$ git checkout $CI_COMMIT_REF_NAME || true
error: pathspec 'mirror-fork' did not match any file(s) known to git
$ git pull origin $CI_COMMIT_REF_NAME
fatal: couldn't find remote ref mirror-fork
Cleaning up file based variables
ERROR: Job failed: exit code 1