Add H237 to HACKING.rst

H237 was added in I7811bbf2ada79e2af988707e0851a2a2e64d3bf7 but wasn't
added to HACKING.rst

Change-Id: I9476748b9919671ee9fdbb50ede91f7e66ba5bac
Merge "Add H237 to HACKING.rst"