README: add github stars from shields.io

[ahmetb]

Since we passed 1k recently, worth celebrating. 🍸

[codecov-io]

Codecov Report

Merging #257 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #257   +/-   ##
=======================================
  Coverage   53.76%   53.76%           
=======================================
  Files          18       18           
  Lines         863      863           
=======================================
  Hits          464      464           
  Misses        342      342           
  Partials       57       57

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[corneliusweig]

🍾
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, corneliusweig

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