DOC: Add a link to the release notes in README.md

[mwaldstein]

Every time there is a new release, I need to go searching for this - not sure that everyone is aware of where these are placed online, seems to make sense to have a link included here.

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[mwaldstein]

I signed it!

[googlebot]

CLAs look good, thanks!

[jshcrowthe]

Thanks @mwaldstein