change(docs): Add private IP address known issue to README

[teor2345]

Motivation

We want to stop using private IP addresses by default in Zebra (#3117), but we might not have time to do it before the first stable release.

Solution

Add it as a known issue to the readme

Review

This is a low priority doc update

Do we also want to add a note about private IPs to the next release changelog, or the stable release changelog?

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?