Update README.md

[kscottz]

Summary

I want to clarify what will be supported in Jazzy. I simply copied the current Rolling support. LMK if that's incorrect.

I also added a note potting to REP-2000 for additional details. No DCO as I did this in browswer really quick. Let me know if you need that too.

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[kscottz]

If we are asking users to upgrade right now, I think we should give guidance on the next ROS LTS as soon as we know so people can make informed decisions about what versions to target. The Jazzy release is just three months and two days away.

[kscottz]

Folded in those changes, the post is live, let's get this merged as soon as we can.