add circle badge

Author: bahmutov

README.md

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-# netlify-plugin-prebuild-example [![Netlify Status](https://api.netlify.com/api/v1/badges/b886d5e8-4be4-4a23-8c65-7601ae479c53/deploy-status)](https://app.netlify.com/sites/netlify-plugin-prebuild-example/deploys) [![renovate-app badge][renovate-badge]][renovate-app]
+# netlify-plugin-prebuild-example [![Netlify Status](https://api.netlify.com/api/v1/badges/b886d5e8-4be4-4a23-8c65-7601ae479c53/deploy-status)](https://app.netlify.com/sites/netlify-plugin-prebuild-example/deploys) [![renovate-app badge][renovate-badge]][renovate-app] [![CircleCI](https://circleci.com/gh/cypress-io/netlify-plugin-prebuild-example/tree/master.svg?style=svg)](https://circleci.com/gh/cypress-io/netlify-plugin-prebuild-example/tree/master)
 > An example site built and tested on Netlify before and after build step using [netlify-plugin-cypress](https://github.com/cypress-io/netlify-plugin-cypress)
 
 This starter demonstrates advanced Cypress configuration options in the [netlify.toml](netlify.toml). Most users can use the other starter, which has simplified configuration options: [netlify-plugin-cypress-example](https://github.com/cypress-io/netlify-plugin-cypress-example).