update readme

[RomanBednar]

OPERATOR_IMAGE_VERSION is now required with the new featuregate approach. If version is not set the operator starter fails with:

cluster failed with : unable to determine features: missing desired version "" in featuregates.config.openshift.io/cluster

Same change as for Azure File operator: openshift/azure-file-csi-driver-operator#70

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RomanBednar

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[jsafrane]

/lgmt
/retest-required

[jsafrane]

/lgtm

[RomanBednar]

/label docs-approved
/label px-approved
/label qe-approved

[openshift-ci-robot]

/retest-required

Remaining retests: 0 against base HEAD 7fd275a and 2 for PR HEAD 894a4d7 in total

[openshift-ci]

@RomanBednar: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-azurestack-csi	894a4d7	link	false	/test e2e-azurestack-csi

Full PR test history. Your PR dashboard.

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[RomanBednar]

/retest-required