Update Openshift installation documentation

[waveywaves]

Add command for adding scc to webhook serviceaccount
in the documentation for installing Tekton Pipelines on Openshift

Changes

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• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Release Note

[waveywaves]

/assign @vdemeester

[tekton-robot]

@waveywaves: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[waveywaves]

/kind documentation

[vdemeester]

/release-note-none

[vdemeester]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[vdemeester]

/test pull-tekton-pipeline-build-tests