added technical debt badge

Author: AllanHoejgaardJensen

README.md

@@ -4,7 +4,7 @@ In every API of a certain size a number of capabilities are used to provide a pa
 
 [![Build status](https://travis-ci.org/openapi-tools/api-capabilities.svg?branch=master)](https://travis-ci.org/openapi-tools/api-capabilities)
 [![Coverage Status](https://codecov.io/gh/openapi-tools/api-capabilities/coverage.svg?branch=master)](https://codecov.io/gh/openapi-tools/api-capabilities)
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+[![Technical Debt](https://sonarcloud.io/api/badges/measure?key=io.openapitools.api%3Acapabilities&metric=sqale_debt_ratio)](https://sonarcloud.io/dashboard?id=io.openapitools.api3%capabilities)
 
 This example contains the following capabilities: