docs: add OpenSSF Scorecard to README

[dalehenries]

What this PR does / why we need it:
Adds the OpenSSF Scorecard to the README

#SecuritySlam

Which issue(s) this PR fixes:
Related to #2274

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[mrueg]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dalehenries, mrueg

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[mrueg]

Thanks for your contribution @dalehenries !

[rexagod]

Thank you for your work here, @dalehenries!

The OpenSSF practices are something we can surely utilize within the repository. However, to better visualize and trigger the checks (IIUC these are only presented as raw json currently), can we add an action for it here as well?