Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Introduction
+
+This repository is one of the main components of the RIHA infrastructure. RIHA is being maintained and developed by Estonian Information System Authority.
+
+## Prerequisits of submitting a change in RIHA-Browser
+
+- Your code is reasonably covered with unit tests
+- Your code has been tested
+- RIHA-Browser's API description (swagger.yaml) has been updated
+
+Updating the API description is automated and should be regenerated before each merge/pull request. API descriptions are generated using a [swagger maven plugin](https://github.com/kongchen/swagger-maven-plugin) which uses annotations in Java code to generate a new desciption. To run the generation, use 
+ `mvn swagger:generate` which will generate a swagger.yaml file in `src/main/resources/static` folder.