Add example use of --openshift-ca argument

[JustinKuli]

We ran into the problem we think is described in https://github.com/openshift/oauth-proxy/issues/73, and found this as a workaround.

If the default ingress certificate is changed in the cluster, then the OpenShift OAuth server certificate might no longer be trusted by default. However, assuming the certificate was changed with this procedure (link), then OpenShift will add the new certificate to a bundle which we can mount into the container (more info on that process here (link).

This PR adds a section describing this configuration in the README, and adds it to the example sidecar configuration.

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[openshift-ci-robot]

Hi @JustinKuli. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[JustinKuli]

/assign @deads2k

[openshift-bot]

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Mark the issue as fresh by commenting /remove-lifecycle stale. Stale issues rot after an additional 30d of inactivity and eventually close. Exclude this issue from closing by commenting /lifecycle frozen.

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/lifecycle stale

[openshift-bot]

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Mark the issue as fresh by commenting /remove-lifecycle rotten. Rotten issues close after an additional 30d of inactivity. Exclude this issue from closing by commenting /lifecycle frozen.

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/lifecycle rotten /remove-lifecycle stale

[openshift-bot]

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Reopen the issue by commenting /reopen. Mark the issue as fresh by commenting /remove-lifecycle rotten. Exclude this issue from closing again by commenting /lifecycle frozen.

/close

[openshift-ci-robot]

@openshift-bot: Closed this PR.

In response to [this](https://github.com/openshift/oauth-proxy/pull/180#issuecomment-751405072): >Rotten issues close after 30d of inactivity. > >Reopen the issue by commenting `/reopen`. >Mark the issue as fresh by commenting `/remove-lifecycle rotten`. >Exclude this issue from closing again by commenting `/lifecycle frozen`. > >/close Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.