add attribution to the Django project in the shell command module

wcs/ctl/shell.py

@@ -14,6 +14,11 @@
 # You should have received a copy of the GNU General Public License
 # along with this program; if not, see <http://www.gnu.org/licenses/>.
 
+# Most of this code is copied from the shell command [1] of the project Django
+# release 1.4. Many thanks to them.
+#
+# [1]: file django/core/management/commands/shell.py
+
 import os.path
 
 from qommon.ctl import Command, make_option