Update Version Compatibility in readme

[Sarah-A]

Ref. #1511

Update Version Compatibility section in readme:

• Add SDK version 13.0 to the compatibility table
• Clarity client's backward compatibility
• Clarify back-porting fixes to older versions
• Some typo fixes

/kind documentation

[linux-foundation-easycla]

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[Sarah-A]

I'm working with my company to sign the CLA. I'll sign it once it's approved.

[tg123]

you can do it personally

[brendandburns]

/lgtm
/approve

Thanks for the PR!

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