Create CONTRIBUTING.md

Author: mohsen1

CONTRIBUTING.md

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+## Pull Requests
+Plase make your pull requests are made to the [**`develop_2.0`**](https://github.com/swagger-api/swagger-ui/tree/develop_2.0) branch at this time.
+
+## Issues
+SwaggerUI uses [SwaggerJS](https://github.com/swagger-api/swagger-js) library for many internal operations. If you see errors in
+[`swagger-client.js`](lib/swagger-client.js) file, you should probably open the issue in [SwaggerJS](https://github.com/swagger-api/swagger-js) repository.
+
+Please open issues related to Swagger specifications in [Swagger Specs](https://github.com/swagger-api/swagger-spec) repository.