HACKING: update HACKING.rst file to include latest changes

Commit 3491cbc0c5 updated a hacking
check but did not update the HACKING.rst file.

Change-Id: Ie9be701f61da93686b79df23f1acf14008e4d486

HACKING.rst

@@ -19,6 +19,8 @@ Neutron Specific Commandments
 - [N327] Python 3: do not use dict.iteritems.
 - [N328] Detect wrong usage with assertEqual
 - [N329] Method's default argument shouldn't be mutable
+- [N330] Use assertEqual(*empty*, observed) instead of
+         assertEqual(observed, *empty*)
 
 Creating Unit Tests
 -------------------