Update HACKING.rst to include N320

Commit 9235ada8c2c250603dc5b299cc08bb7a982d4fc6 did not add in
the updated hacking rule.

Change-Id: If2daf6d2e7008c36d0aba6270ac034522dcb2e2b
Closes-bug: #1325812
Merge "Update HACKING.rst to include N320"