create Contributing md

Author: csviri

CONTRIBUTING.md

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+## Releases
+To release the project to maven central, create a new release in github. 
+Set both tag and name ing a format: `v[major].[minor].[patch]`, sample: `v1.2.23`. 
+And follow classic semver guidlinces. Note that the version is in the pom files is 
+calculated based on tag not by release name in the release workflow.