Add information about the Istio CNI plugin in the README

[DnPlas]

Context

Since we are enabling the Istio CNI plugin by default in future versions of istio-pilot we must document what will be the differences and what actions must be taken by users during and after upgrading the charm from previous versions. The README should cover:

• Basic information about the Istio CNI plugin and what difference it makes
• How to deploy the charm on a fresh install
• Information about the new configuration options and how to properly set them. This section must be clear about the cni bin dir and cni conf dir values and how to get them, plus giving some examples.
• Information about the upgrade process and what is required

What needs to get done

A section in the istio-pilot README to document all what's described above.

Definition of Done

An approved PR that introduces the changes to the README.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5176.

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