Create CONTRIBUTING.md

Author: alexv-smirnov

CONTRIBUTING.md

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+# Notice to external contributors
+
+## Common
+
+YDB is a free and open project and we appreciate to receive contributions from our community.
+
+## Contributing code changes
+
+If you would like to contribute a new feature or a bug fix, please discuss your idea first on the GitHub issue.
+If there is no issue for your idea, please open one. It may be that somebody is already working on it,
+or that there are some complex obstacles that you should know about before starting the implementation.
+Usually there are several ways to fix a problem and it is important to find the right approach before spending time on a PR
+that cannot be merged.
+
+## Provide a contribution
+
+To make a contribution you should submit a pull request. There will probably be discussion about the pull request and,
+if any changes are needed, we would love to work with you to get your pull request merged.
+
+## Other questions
+
+If you have any questions, please mail us at [email protected].