buildextend-legacy-oscontainer.py: add header for context
That helps other readers understand why this file is a near duplicate of
oscontainer-deprecated-legacy-format.py.
#
# This command manipulates those images.
+# This file was forked from src/oscontainer-deprecated-legacy-format.py and is
+# now only used to build the legacy oscontainer via supermin.
+
import gi
gi.require_version('OSTree', '1.0')
