Need to document zeroDowntimeEnabled

[bonickle]

Our update to 3.71 was failing due to the new "zeroDowntimeEnabled" forcing the update strategy to "on delete". Our error can be found below. We fix this by setting "zeroDowntimeEnabled" since we do not want the "on delete" behavior in our environment. The issue is there was no document on this change, explaining what this was for and why. This needs to be documented in the readme and should have in release notes for 3.70.1.

Error: UPGRADE FAILED: release nexus failed, and has been rolled back due to atomic being set: cannot patch "nexus-nxrm-ha" with kind StatefulSet: StatefulSet.apps "nexus-nxrm-ha" is invalid: spec.updateStrategy.rollingUpdate: Invalid value: apps.RollingUpdateStatefulSetStrategy{Partition:0, MaxUnavailable:(*intstr.IntOrString)(nil)}: only allowed for updateStrategy 'RollingUpdate'

This is the commit in https://github.com/sonatype/nxrm3-ha-repository/pull/92/files