Hacking: add rule number to HACKING.rst

Commit ac8bce63f8a7ec8a2ebb214ea7f86ee4f8adecae added hacking rules
N319. This was not documented in the file HACKING.rst.

Change-Id: Ibf7917aaada88db5afe1387859a387481ec05118
Closes-bug: #1313322