Updates and extends client cert documentation

[llb4ll]

Updates and extends the documentation about enabling client
certificate authentication.
Extends the provided example by updating the yaml definition
and adding setup instructions to the README.

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

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This pull-request has been approved by: llb4ll
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[k8s-reviewable]

This change is 

[llb4ll]

Second try CLA

[llb4ll]

/retest
/meow

[k8s-ci-robot]

@llb4ll:

In response to this:

/retest
/meow

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