updating readme with the last two releases

[pritidesai]

Changes

Added two new entries in the release table for 0.39 and 0.38.3 with all the relevant links to the docs and examples.

/kind documentation

Submitter Checklist

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• Has Docs included if any changes are user facing
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Release Notes

Updating README to include last two releases details - 0.39 and 0.38.3.

[jerop]

/lgtm

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