Add documentation for Horizon operator

[bshephar]

This change populates the README file with usable information and instructions for using, and also contributing to this operator.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bshephar

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[openshift-ci]

@bshephar: 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/precommit-check	e0b8c3e	link	false	/test precommit-check

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[kajinamit]

/lgtm