Add Gerrit v3.3 downgrade to v3.2 instructions

Gerrit v3.3 schema changes are so little that they can be reverted
for downgrading to v3.2.

Add the specific instructions to the release notes, so that Gerrit
admins can refer to them for the rollback section of their upgrade
cutover planning.

Change-Id: I90271b97c20cca3f5131081882ff98ab50acfe81
1 file changed