README: add mention to custom_cni

[j4m3s-s]

What type of PR is this?

/kind documentation

What this PR does / why we need it:
Documents presence of custom_cni CNI support in Kubespray.
Does this PR introduce a user-facing change?:

Add a mention for custom_cni in CNI list

[k8s-ci-robot]

Hi @j4m3s-s. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[oomichi]

Cool, thanks for wrting it down.

/ok-to-test
/approve

[floryut]

@j4m3s-s Thank you for the PR
/lgtm
/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, j4m3s-s, oomichi

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