Add logo

Author: RomanHotsiy

README.md

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 # generator-openapi-repo [![NPM version][npm-image]][npm-url] [![Dependency Status][daviddm-image]][daviddm-url]
 > Yeoman generator for OpenAPI(fka Swagger) repo to help you share spec for your API
 
+<p align="center"> ![logo]('./logo.png') </p>
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 ## Why?
 There are a few advantages in hosting your API specification + docs on GitHub:
  - Community engagement (PR's and issues)